Terminology and Morphology

The purpose of this page is twofold: to present a very brief introduction to the morphology of the creatures found in the Trenton Group through sketches and diagrams and also to define some of the more advanced terms that are found in the species descriptions within the "Fauna" section of this site.

Click on a category below to be taken to its individual section immediately, or scroll down the page to view all sections.

Phylum Arthropoda

Phylum Brachiopoda

Phylum Bryozoa(Under Construction)

Phylum Echinodermata

Phylum Mollusca

Misc. Groups




Images taken from Howe, Kelsing, and Scott ,1961 and Scott, 1961

Some very basic ostracode morphology can be seen in the two images below. The image on the left shows the internal anatomy of a Recent ostracode. The image on the right displays some simple external features that may be present on the valves.


      Internal Anatomy ( from Howe, Kelsing, and Scott, 1961)   External Anatomy (from Scott, 1961)    
Internal Anatomy
External Features


Definitions of terms taken from Moore, 1961

  • Adductor muscle scar -Impression on valve interior of muscle serving for closure of valves, generally located in front of mid-length.
  • Border (or margin) – Periphery of carapace or valve as seen in lateral view.
  • Carapace – Protective covering of ostracode soft parts, including appendages, forming two nearly symmetrical valves joined together by hinge along dorsal border; mostly hard calcareous but soft and uncalcified in most Archeocopida and many Myodocopida.
  • Contact margin – Edge part of valves exclusive of hinge, in contact when valves are closed, its distal limit comprising free edge.
  • Equivalved - Valves having the same size and shape.
  • Denticulate - Bearing a series of small spinelike or toothlike projections.
  • Free edge – Line of contact between closed valves except along hinge line; marks distal limit of contact margin and may lie inside free margin.
  • Free Margin – Anterior, ventral, and posterior parts of border where valves are not held together by hingement.
  • Furrow – Shallow groove on valve surface.
  • Granulose – Surface ornamentation of valves consisting of more or less closely spaced minute protuberances, generally without distinct pattern, like grains of sandpaper.
  • Hinge - Part of valves along or near dorsal border serving for articulation.
  • Hinge line – Line along which valves articulate, seen when carapace is complete; it may coincide with dorsal border or be depressed below it.
  • Lobe – Rounded major protuberance of valve surface, generally best developed in dorsal part of carapace; also used for part of hinge tooth (projection of hinge area aiding in articulation of valves).
  • Margin – See border.
  • Nonsulcate - Valve surface evenly elevated, unilobate, lacking sulcus.
  • Node – Protuberance of intermediate size on valve surface, clearly distinct from lobes.
  • Punctate – Surface bearing many minute depressions resembling pin pricks.
  • Sulcus – More or less prominent groove or trench on valve surface trending dorsoventrally and generally best developed in dorsal half of carapace; in some genera may be reduced to faint depression.
  • Trisulcate – Valves having three sulci (S1 S2 S3) and four lobes.
  • Valve – One of the two halves of carapace, hinged at upper (dorsal) edge, classed as left valve and right valve.
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These two images are labeled with some of the basic morphology of the trilobite exoskeleton. The diagram to the left is based on an Isotelus gigas specimen and to the right, a Ceraurus pleurexanthemus specimen. Both are found in the Trenton Group at West Canada Creek.


      Trilobite morphology   Trilobite morphology  
Exoskeletal Morphology
Details of the Cephalon


Definitions of terms taken from Whittington and Kelly, 1997

  • Axial furrow – Groove on external surface formed by fold in exoskeleton that outlines the axial region of cephalon, thorax, and pygidium.
  • Axial lobe – Median region of dorsal exoskeleton, bordered by axial furrow including preglabellar furrow.
  • Axial ring – Central portion of thoracic segment or of a segment of pygidium, bounded laterally by axial furrow. May be subdivided by intra-annular furrow into preannulus and postannulus.
  • Border - Outer dorsal portion of cephalon and pygidium and outer ventral portion of hypostome, usually bounded by border furrow (has been referred to as rim, marginal rim, or marginal limb, but the term rim is restricted here to narrow peripheral ridge of fringe of Trinucleidae, etc.).
  • Border furrow – Furrow defining adaxially border of cephalon, pygidium, and hypostome (syn., marginal furrow).
  • Cephalon (pl., cephala) – Anterior tagma composed in trilobites of a number of fused somites ( transverse division of arthropod body), applied by authors to anterior portion of exoskeleton separated from remainder of exoskeleton by an articulation (syn., cephalic region, head, head shield).
  • Cranidium (pl., cranidia) – Central portion of exoskeleton of cephalon, bounded laterally by facial suture.
  • Dorsal shield – Mineralized dorsal cuticle extended onto ventral side as doublure and hypostome.
  • Doublure – Reflexed continuation of dorsal exoskeleton onto ventral surface.
  • Facial suture – Suture bounding adaxial margin of eye surface, which had anterior and posterior branches (sections), each of which may extend over the border and on to the doublure; suture separates free cheek from fixed cheek. Posterior branch extends to inner margin of doublure, anterior branches may meet medially on dorsal or ventral surface of exoskeleton or may join extremity of rostral suture. Facial suture may be present when eye is absent.
  • Fixed cheek – Portion of cranidium abaxial to axial and preglabellar furrows and imaginary sagittal line bisecting frontal area.
  • Free cheek – Portion of cephalon abaxial to facial suture.
  • Genal angle – Posterolateral corner of cephalon.
  • Genal spine – General term for hollow, posteriorly directed extension of border and doublure at genal angle, forming pointed projection.
  • Glabella – Axial portion of cephalon, bounded by axial and preglabellar furrows.
  • Granule – Minute protuberance from exterior surface of exoskeleton, smaller than tubercle or pustule.
  • Holaspid (holaspid period) – Period of growth after specific number of thoracic segments had been attained.
  • Holocroal eye – Compound eye consisting of numerous adjoining lenses covered by a continuous cornea.
  • Hypostome – Ventral sclerite (portion divided by an articulation or by a suture) situated beneath anterior portion of glabella.
  • Interpleural furrow – Transverse groove extending from axial furrow across pleural region of pygidium, indicating boundary of fused pleurae (syn., interpleural groove, rib furrow).
  • Lateral glabellar furrow – Bilaterally symmetrical pairs of narrow grooves on external surface formed by fold in exoskeleton, extending partway across glabella from (or near) axial furrow. Furrows vary in length, depth and direction; they may be short, limited to pits in or close to axial furrow, or isolated from axial furrow. When three pairs are present, they have been referred to as anterior, median, and basal (syn., posterior glabellar furrow, preoccipital glabellar furrow). Numbering from the back forward is preferred: occipital as S0, preoccipital as S1, and continued forward as S2, S3, etc.
  • Lateral glabellar lobe – Portion of glabella outlined and more or less separated by successive lateral glabellar furrows, may or may not be inflated. Numbering is from posterior end forward: L1, L2, etc. In lichids it is considered that L1 is subdivided into posterior L1a and anterior L1b, and similar subdivisions are recognized in odontopleurids. When three pairs are present they have been referred to as anterior, median, and basal lobes.
  • Lunette – Raised cresentic area on inner (parietal) surface of exoskeleton at abaxial margin of axial furrow, where glabella is narrowest and approximately in transeverse line with eye lobe; characteristic of Illaenidae.
  • Margin (frontal/posterior) – Distal edge of portion of exoskeleton.
  • Occipital furrow – Transverse furrow (S0) that isolates occipital ring from remainder of glabella.
  • Occipital ring – Axial region of most posterior segment of cephalon, bounded at sides by axial furrows, at front by occipital furrow, and at back by posterior margin. Considered part of glabella in all trilobites.
  • Ontogeny - The growth cycle.
  • Palpebral lobe – Protruding subsemicircular flange of fixed cheek bounded distally by palpebral suture (portion of facial suture bounding distal edge of palpebral lobe). Forms with the visual surface of the eye to form the eye lobe.
  • Pleural furrow – Groove in external surface of thoracic pleuron or pleural region of pygidium, formed by fold in exoskeleton.
  • Pleural lobe – Lateral portion of thorax or pygidium abaxial to axial furrow.
  • Pleuron ( pl., pleurae) – Lateral portion of thoracic segment.
  • Preglabellar furrow – Portion of axial furrow outlining front of glabella.
  • Pustule – Small, rounded elevation on any part of external surface of axoskeleton. See tubercle.
  • Pygidium (pl., pygidia) – Posterior tagma in trilobites, composed of fused somites (transverse division of arthropod body), applied by authors to posterior portion of the exoskeleton separated from the thorax by articulation.
  • Schizocroal eye – Eye with visual surface consisting of number of biconvex lenses, rounded or polygonal in outline, each lens covered by individual cornea and separated from others by sclerotic walls.
  • Subquadrate – Nearly or approximately square.
  • Thorax – Region of trilobite body between cephalon and pygidium in which successive somites (transverse division of arthropod body) are articulated; also used to refer to exoskeleton region, the segments of the thorax.
  • Tubercle – Small, knoblike prominence on external surface. See pustule.
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Images taken and/or modified from (moving left to right) Williams and Rowell, 1965a and Williams et. al., 1997a (combined picture), Williams and Rowell, 1965b, Shrock and Twenhofel, 1953, Williams et. al., 1997b.

The following diagrams and sketches display some important brachiopod morphology.


      Brachiopod Anatomy (modified from Williams and Rowell, 1965a and Williams et. al. 1997)   Brachiopod Symmetry and External Features (from Williams and Rowell, 1965b)    
Brachiopod Anatomy and Muscle Arrangement Based on (top) Terebratulina and (bottom) Calloria inconspicua
External Features and Symmetry (1 posterior view, 2 left lateral, 3 dorsal, 4 dorserolateral)
      Brachiopod Convexity (from Shrock and Twenhofel, 1953)   Brachiopod Spiralia (modified from Williams et. al. 1997)    
Different Forms of Convexity for Brachiopod Shells.
Various Shapes of Spiralia (1 a spiriferid, 2 Protozyga, 3 a zygospirid, 4 a koninckinid with apices directed ventrally, 5 an atrypid with apices directed dorsally)


Definitions of terms from Williams and Brunton, 1997.

  • Acuminate – Phase in loop (support for lophophore consisting of secondary shell material) development with laterally bowed, descending lamellae extending from crura but otherwise unsupported and uniting anteromedially to form an echmidium (spear shaped plate formed in the ontogeny of loop in Paleozoic terebratulid brachiopods).
  • Adductor muscles – Muscles that contract to close shell. Inarticulated brachiopods two adductor muscles, each divided dorsally, are commonly present to produce single pair of scars located between diductor (muscles that open the shell) impressions in ventral valve and two pairs (anterior, posterior) in dorsal valve. In inarticulated brachiopods two pairs of adductor muscles (anterior, posterior) are commonly present, passing almost directly dorsoventrally between valves.
  • Alate – Having winglike extensions at cardinal extremities.
  • Anacline - see inclination of cardinal area or of pseudointerarea
  • Apex – First formed part of valve around which shell has grown subsequently (term usually restricted to valves having this point placed centrally or subcentrally).
  • Apsacline – see inclination of cardinal area or of pseudointerarea
  • Auriculate – Opening bounded by deltidial plates bearing external rims or winglike extensions.
  • Beak – Extremity of umbo (apical portion of either valve), commonly pointed.
  • Biconvex – Both valves convex.
  • Bifurcate – Lophophore support in form of Y-shaped median septum.
  • Brachial valve – See dorsal valve.
  • Brachiophores – Blades of secondary shell projecting from either side of notothyrium (opening bisecting dorsal cardinal area or pseudointerarea) and forming or in close association with inner socket ridges.
  • Capillate – Having very fine radial ridge on outer surface of shell.
  • Cardinal angles – Angle between hinge line and posterolateral margins of shell.
  • Cardinal process – Blade or variably shaped boss of secondary shell situated medially in posterior end of dorsal valve and serving for separation or attachment of paired diductor muscles (muscles that open valves).
  • Chilidium – Crescentic plate covering apex of notothyrium (opening bisecting dorsal cardinal area or pseudointerarea), commonly convex externally and extending for variable distances ventrally over proximal end of cardinal process and chilidial plates when present.
  • Commissure – Line of junction between edges or margins of valves.
  • Concavoconvex – Dorsal valve concave; ventral valve convex.
  • Convexoconcave – Dorsal valve convex; ventral valve ventral valve concave.
  • Convexoplane – Dorsal valve convex; ventral valve plane.
  • Costa (pl., costae) – First formed radial ridge on external surface of shell most commonly originating at the junction between the brephic and neanic shell (juvenile stages of growth development). [Also but ambiguously used for any coarse rib, without reference to origin. Costella is a fine rib, amd capilla is a very fine rib. This usage gives no indication of the nature of radial ornamentation and any quantitative definitions, related to the incidence of ribs at the margins of shell irrespective of their sizes, are of no value. Thus in brachiopods ornamented by ribs that increase in wave length during growth, shells of the same species could be described as capillate, costellate, or costate according to their size.]
  • Costate – Shell radially ornamented exclusively by costae.
  • Costella (pl., costellae) – Radial ridge on external surface of shell originating later than costa bifurcation of existing costa or costella or by intercalation between earlier-formed ribs.
  • Costellate - Having costellae.
  • Crenulations – Small, protruding ridges that alternate with complementary sockets located along cardinal margin or hinge line of both valves; also small processes on posterior surfaces of dental sockets fitting into accessory sockets in hinge teeth.
  • Crura – Paired processes extending from cardinalia (structures of secondary shell in posteromedian region of dorsal valve that deal with support of lophophore, muscle attachment, and articulation) or septum to give support to posterior end of lophophore; distal ends may also be prolonged into primary lamellae of spire or descending lamellae of loop.
  • Crural bases – Parts of crura united to hinge plates or socket plates and separating inner and outer hinge plates when present.
  • Crural pits – Cavity near floor of dorsal valve separating brachiophore plate (basal part of brachiophore that join floor of valve) and fulcral plate in some orthides.
  • Crural plates – Plate extending from inner edge of outer hinge plate or crural base to floor of dorsal valve; may fuse medially with counterpart to form septalium.
  • Cynocephalous – Sharply folded dorsal valve.
  • Deltidial plates – Two plates growing medially from margins of delthyruim (aperture bisecting ventral cardinal area or pseudointerarea, commonly serving as pedicle opening), partly or completely closing it.
  • Dental cavities – Anteriorly expanding cavity, presumably occupied by evagination of outer epithelium secreting the tooth of some atrypoids.
  • Dental lamellae – See dental plates.
  • Dental plates – Variably disposed plates of secondary shell underlying hinge teeth and extending to floor of ventral valve.
  • Dorsal valve – Valve that invariably contains any skeletal support for lophophore and never wholly accommodates pedicle; commonly smaller than ventral valve and with distinctive muscle-scar pattern.
  • Dorsibiconvex – Dorsal valve more convex than ventral valve.
  • False area – Previously used for any poorly defined cardinal area or pseudointerarea.
  • Filum (pl., fila) – Fine concentric ridge of variable persistence ornamenting external surface of shell.
  • Fold – Major elevation of valve surface, externally convex in transverse profile and radial from umbo.
  • Fulcral plates – Small plate raised above floor of dorsal valve extending between posterior margin and brachiophore plate (basal part of brachiophore that join floor of valve) or inner socket ridge and socket plate and forming floor of socket.
  • Geniculate – Abrupt and more or less persistent change in direction of valve growth producing angular to sharply rounded bend in lateral profile.
  • Gibbous - A prominent swelling to one side.
  • Growth lines – Concentric line on outer surface of shell formed when forward growth of shell temporarily ceased.
  • Hinge line – Straight posterior margin of shell parallel with hinge axis; previously used as synonym of cardinal margin.
  • Hinge teeth – Two principle articulating processes situated at anterolateral margins of delthyrium (aperture bisecting ventral cardinal area or pseudointerarea, commonly serving as pedicle opening) and articulating with dental sockets in dorsal valve.
  • Hypercline – See inclination of cardinal area or of pseudointerarea.
  • Inclination of cardinal area or of pseudointerarea – Commonly used terms to describe the condition of either valve based on convention of viewing specimen in lateral profile with beaks to left and dorsal valve uppermost, referring cardinal area to its position within one of four quadrants defined by commissure plane and plane normal to it and symmetry plane, touching base of cardinal areas. Cardinal area lying on continuation of commissure plane is orthocline. Moving clockwise, cardinal area in first quadrant is weakly to strongly apsacline; at 90 degrees to orthocline it is catacline; and continuing counterclockwise into bottom right quadrant cardinal area is weakly to strongly procline.
  • Jugum – Medially placed structure of secondary shell connecting two primary lamellae of spiralia.
  • Lamella (pl., lamellae) – Sheetlike extension of primary and even underlying secondary shell deposited by retractile mantle (prolongation of body wall, secretes shell) margin on external shell surface; also used for calcareous ribbon comprising spiralium and coiled extension of arm of jugum.
  • Lamellose – Having lamellae.
  • Listrium – Plate in some discinids closing anterior end of pedicle opening that has migrated posteriorly.
  • Lophophore – Feeding and respiratory organ with tentacles, symmetrically disposed around mouth, typically suspended from anterior body wall but may be attached to dorsal mantle (prolongation of body wall, secretes shell); occupies mantle cavity (anterior space between valves bounded by mantle and anterior body wall).
  • Muscle field – Area of valve in which muscle scars are concentrated.
  • Muscle scars – More or less well-defined impression or elevation on valve representing final site of attachment of muscle.
  • Myophore – Differentiated site of diductor muscle attachment on cardinal process, consisting of ridged myotest (muscle scar shell).
  • Oblique muscle – Variable sets of muscles in inarticulated brachiopods responsible for rotational and longitudinal movements of valves.
  • Orthocline - See inclination of cardinal area or of pseudointerarea.
  • Parvicostellate – Costellae numerous, arising entirely by intercalation between widely spaced costae.
  • Pedicle – Variably developed, cuticle-covered, stalklike appendage commonly protruding from ventral valve that adjusts position of shell relative to external environment.
  • Pedicle foramen – Subcircular to circular perforation of shell through which pedicle passes.
  • Pedicle groove – Subrectangular groove dividing ventral pseudointerarea medially and affording passage for pedicle in many lingulides.
  • Pedicle notch – Small, subtriangular depression, posteromedially placed on limbus (flattened inner margin of inarticulate valve) of paterulids, probably functioning as pedicle groove.
  • Pedicle opening – Variably shaped aperture in shell through which pedicle emerges.
  • Pedicle tube – Internally directed tube of secondary shell continuous with margin of pedicle foramen and enclosing proximal part of pedicle.
  • Pedicle valve – See ventral valve.
  • Planoconvex – Dorsal valve flat; ventral valve convex.
  • Plication – Major undulation of commissure, reflected on shell interior, with crest directed dorsally; commonly but not invariably associated with dorsal fold and ventral sulcus.
  • Propareas – Pair of subtriangular halves of psuedointerarea divided medially by various structures of inarticulated brachiopods.
  • Psuedodeltidium – Single, convex, or flat plate affording variability complete cover of delthyrium (aperture bisecting ventral cardinal area or pseudointerarea, commonly serving as pedicle opening) but invariably closing apical angle when foramen is supra-apical or absent and always dorsally enclosing apical foramen.
  • Pseudointerarea – Somewhat flattened, posterior sector of shell of some inarticulated brachiopods secreted by posterior sector of mantle (prolongation of body wall, secretes shell) not fused with that of opposite valve.
  • Puncta (pl., punctae) – Perforation penetrating shell to connect with periostracum (organic external layer of shell) and occupied by caecum as in cranioids; any perforation apparently penetrating fossil shell and large enough to accommodate caecum.
  • Punctate – Having punctae.
  • Quincuncial (quincunx arrangement) – An arrangement of five punctae with one at each corner of a rectangle or square and one at the center.
  • Raduliform – Hook shaped or rodlike crura that arise on ventral side of hinge plate and project toward ventral valve.
  • Rectimarginate – Having planar anterior commissure.
  • Ruga (pl., rugae) – Concentric or oblique wrinkling of external shell surface.
  • Septalium – Troughlike structure of dorsal valve between crural bases, consisting of crural plates (or homologues) fused medially and usually supported by median septum, but may be unsupported or sessile; does not carry adductor muscles. See crural plates.
  • Septum (pl., septa) – Relatively long, narrow elevation of secondary shell, commonly bladelike; indicated within underlying floor of valve by persistent high, narrow deflections of shell fabric originating near primary layer (outer , mineralized shell layer beneath the exterior organic layer).
  • Sinus – Major undulation of commissure with crest directed ventrally, commonly but not invariably associated with ventral fold and dorsal sulcus; also used irrespective of commissure, as a synonym of sulcus.
  • Socket – Excavations in posterior margin of dorsal valve for reception of hinge teeth.
  • Socket plate – Pair of plates supporting inner socket ridges, attached to cardinal process, and resting ton floor of dorsal valve of orthotetoids and some orthoids; also pair of plates defining sockets of atrypid dorsal valve, confined by iner socket ridges medially and normally supported by thickened shell deposit.
  • Socket ridge – Linear elevations of secondary shell extending laterally from cardinal process and bounding anteromedian margin of dental sockets.
  • Spiralia – Pair of spirally coiled lamellae composed of secondary shell and supporting lophophore.
  • Spiralophous – Having lophophore in which brachia are spirally coiled and bear single row of paired tentacles.
  • Striae – Fine grooves or incisions.
  • Sulcate – Form of alternate folding with dorsal valve bearing median sulcus and anterior commissure median sinus.
  • Sulcus – Major depression of valve surface, externally concave in transverse profile and radial from umbo (apical portion of either valve containing the beak).
  • Truncated – Ventral umbo (apical portion of either valve containing beak) or articulated brachiopods with beak abraded due to pedicle movement and foramen in transapical position.
  • Umbonal muscle scar – Scar left by single muscle occurring in some lingulides, thought to be homologous with posterior adductors; consists of two bundles of fibers posteriorly and slightly asymmetrically placed.
  • Uniplicate – Form of alternate folding with ventral valve bearing median sulcus and anterior commissure median plica (plication).
  • Valve – One of two separate halves that make up the brachiopod shell.
  • Vascula media – Pair of mantle (prolongation of body wall, secretes shell) canals in either valve, medially located, projecting anteriorly from body wall.
  • Ventral valve – Valve through which pedicle commonly emerges, usually larger than the dorsal valve and invariably containing teeth when present.
  • Ventribiconvex – Both valves convex, ventral valve more strongly so than dorsal.
  • Visceral area – Part of shell enclosing body cavity.


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Images modified from figure in Spencer and Wright, 1966

Images below compare the morphology between recent asterozoans.


      Asteroid Morphology (modified from  Spencer and Wright, 1966)   Ophiuroid Morphology (modified from  Spencer and Wright, 1966)    
Recent Asteroid morphology of Anthenea flavescens
Recent Ophiuroid morphology of Ophiura


Definitions of terms taken from Spencer and Wright, 1966

  • Adambulacral – Ossicle of series on oral surface of ray, next to ambulacrals.
  • Ambulacral – Ossicle of axial skeleton, one of double series of opposite or alternate ossicles formed along radial water vessel that constitutes axis of ray or arm; also pertaining to series of ambulacral ossicles.
  • Ambulacral groove – Axial depression along oral surface of ray that is roofed by series of ambulacral ossicles.
  • Ampulla (pl., ampullae) – Dorsal saclike part of tube foot, either seated externally in cupule or internally and connecting with podium (external extension of tube foot) through podial opening; ampullae may be single or double.
  • Arm – Radial extension of the body surrounding axis consisting of ambulacra (ossicle of axial skeleton); arms may be distinct from disk or not.
  • Axillary – In axil (an angle formed by junction of rays or arms with straight sides); generally applied to single large ossicle in axils of certain asteroids.
  • Carinal – Ossicle of a series along mid-line of aboral surface of ray, in line with primary radial if present.
  • Disk – Central part of body, more or less distinctly separable from arms.
  • Dorsolateral – Ossicle of series on aboral surface of ray, between carinals and marginals (syn., adradial); also directed toward axis of ray.
  • Granulate – Covered with granules. Granules are minute, more or less spherical skeletal elements situated on the surface of ossicles, generally in pits or distributed in covering skin.
  • Inferomarginal – Ossicle of a series along the oral edge of arms or disk or both; in origin part of the adaxial skeleton.
  • Interbrachial – Between the arms; applied to margin or surface of disk or to internal structures.
  • Interradial – Indicating position midway between axis of adjacent rays or area between such rays.
  • Madreporite – Spongy or sievelike ossicle that serves as inlet to the water vascular system; it is located interradially, lateral in some primitive asterozoans, but on the aboral surface in most asteroids and a few ophiuroids and on the oral surface in other phiuroids.
  • Marginal – Ossicle of a series along the edge of the body.
  • Ossicle – Any individual calcified element of skeleton, but normally used for larger of such elements.
  • Papula (pl., papulae) – Short protuberance of integment between ossicles of aboral or oral surface of asteroids that functions as external gill.
  • Papular opening – Gap between ossicles for protrusion of papula.
  • Paxilla (pl., paxillae) – Ossicle of extraxial skeleton with shaft surmounted by tuft of spinelets.
  • Peristomal ring - Plates surrounding the opening for mouth.
  • Podial opening – Passage between ambulacrals for passage of tube foot.
  • Radial – Prominent ossicle on aboral surface of asteroids, in line with mid-line of arm, forming part of primary circlet; commonly used in older literature for any ossicle in series with primary radial (see carinal); also applied to organs extending along arms.
  • Ray –Segment of the body that includes one ambulacral axis.
  • Spine – Sharp or blunt, short or long skeletal element, attached to ossicle by muscle.
  • Superomarginal – Ossicle of series along edge of disk or arms or both, above series of inferomarginals.
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Definitions of terms taken from Kelsing, 1967

  • Ambulacral groove – Groove through which food was conveyed from brachioles to mouth, covered by ambulacral covering plates; in some cystoids lying within flooring plates as zigzag trough, but in others lying directly in surface layer of thecal plates or concealed by a “tegmen” (structure which covers mouth and ambulacral groove).
  • Ambulacrum – Structure in which food was conveyed from brachioles to mouth, always on surface of theca, never erect, exothecal. Five ambulacra in many cystoids, but number may be reduced to 2. Length of ambulacra varies greatly, from very short conduits, where brachioles are clustered near the mouth, to medium, where brachioles are limited to crown of theca, to very long, in some extending to base. Each ambulacrum consisting of ambulacral groove enclosed by biserial covering plates; some ambulacra provided with flooring plates, others with groove entrenched into thecal surface.
  • Arm - Major element in food-gathering structure in some cystoids. Has biserially arranged plates bearing biserial pinnules.
  • Base – Aboral part of theca, by some restricted to columnar facet (face on columnals allowing movement) but by others expanded to include thecal plates and basal circlet or aboral circlets.
  • Brachioles – One of erect structures by which food is gathered and transmitted to ambulacrum. Each brachiole composed of biserial plates and unbranched. Two rows of dorsal plates incude brachiole groove, and 2 rows of tiny ventral plates roof it over.
  • Calyx (pl., calyces) – Part of theca excluding tegmen (structure which covers mouth and ambulacral groove).
  • Column – Stemlike structure attached to aboral end of most cystoids, fairly rigid in Caryocrinites but in most other forms having proximal section flared and flexible. Column presumably used for anchoring cystoid, distal end known to be variously modified in some species.
  • Columnal – Unit of ossicle composing column. Columnals in many species varying according to position in column, distal columnals longer, cylindrical, with narrow central canal, and proximal columnals shorter, ringlike, with wide lumen. Certain proximals strongly modified in some cystoids.
  • Crenulate - Minutely scalloped.
  • Food grooves – See ambulacral groove.
  • Holdfast – Distal structure of column that helps to anchor creature in place.
  • Nodose – Having a nodular appearance.
  • Ossicle - Any calcified piece of exoskeleton.
  • Pectinirhombs – Type of pore rhomb (group of perforations in thecal plates) found in Glyptocystitida, with units or dichophores terminating in perforations through thecal plates, either as round pores or, more commonly, as slits. If slits from one plate are continuous with those of another, pectinirhomb is conjunct; if slits from one plate are separated from those of other by slit free area, pectinirhomb is disjunct; if each dichophore has line of slits in each plate, instead of terminal slit only, pectinirhomb is multidisjunct; and if slits in one half-rhomb are surrounded by rim, pectinirhomb is montidisjunct.
  • Periproct – Major thecal tract containing anal pyramid and, in many species, additional plates; extreme development attained in Pleurocystitidae, in which periproct nearly fills one side of compressed theca. In many Callocystitidae, periproct is filled by small anal pyramid and surrounding ring of auxiliary plates.
  • Pinnule - Branch of the food gathering system of armed cystoids, it differs from pinnules of crinoids in that it is biserial. It is homologous to simple brachioles in other cystoids.
  • Plate – Any calcareous ossicle, normally flat, in cystoid; by some used only for thecal plates, but all calcareous bodies formed serve as framework of support for soft parts and constitute plates.
  • Ray - See ambulacrum.
  • Stem – See column.
  • Suture – Area of contact between two plates.
  • Theca – Enclosure of plates in which body of cystoid was housed.
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Image from Ubaghs, 1978

Basic crinoid morphology is labeled on the sketch below.


          Crinoid Morphology (from Ubaghs, 1978)                
Crinoid Morphology based on Calpiocrinus intermedius.


Definitions of terms taken from Moore et al., 1978

  • A ray – Anterior ray located opposite the posterior interray designated CD, generally wider than others and containing anus or anal plates; includes radial and succeeding brachitaxes with intervening plates as well as pinnules. Other rays designated B, C, D, and E in clockwise order from A, viewing adoral side of theca.
  • Aboral cup – Part of aboral side of body between origins of free arms and column.
  • Anal sac – Variously shaped, generally inflated and strongly elevated part of tegmen as developed among inadunate crinoids.
  • Anal tube - Conical to cylindrical structure, usually of considerable height, bearing anal opening at its summit, typically developed in many camerate, flexible, and articulate crinoids.
  • Arm – Radial evagination of body above radial, normally extending upward or outward from theca, containing coelomic canals, and composed of pinnulate or nonpinnulate brachitaxes. Restricted by some authors to undivided distal branches.
  • Axial canal – Longitudinal passageway for axial cord penetrating columnals, cirrals, thecal plates, arms, and pinnules, generally but not invariably located centrally; may be simple or multiple with main canal accompanied by smaller accessory ones.
  • Axillary - Brachials (any plate above the radials or biradials) that support two arm branches.
  • Axil-arm – Arm including its branches borne by any branch of main-axil as seen in calceocrinids but excluding terminal branchlet termed omega-ramule.
  • Basal – Any plate of circlet next proximal to radials, each basal typically in interradial position.
  • Bilateral heterotomy - Type of arm branching characterized by occurence of ramuli on either side of main arm.
  • Brachitaxis – Series of brachials (any plate above the radials or biradials) extending from radial or biradial to and including primaxil or in arms lacking axillaries to distal extremity of arm; likewise brach series extending from any axiallary to and including next one or to distal extremity of arm or branch.
  • Column – Series of segments composing stem; excludes holdfasts.
  • Columnals – Individual ossicle (calcified segment of skeleton) of crinoid stem.
  • Crown – Whole crinoid exclusive of stem.
  • Cup – See aboral cup.
  • D ray – See A ray.
  • E ray – See A ray.
  • Food grooves – Simple or branched furrow running in adoral surface of tegmen, arms, and pinnules; serves to convey food to the mouth.
  • Heterotomous branching - Arm characterized by division into unequal branches.
  • Holdfast – Any structure at distal extremity of column serving for fixation.
  • Infrabasals – Any plate of proximal circlet in aboral cup having two circlets of plates below radials.
  • Isotomous branching - Characterized by division of arm in equal branches.
  • Nodose – Having a nodular appearance.
  • Non-pinnulate - Without pinnules.
  • Pinnulate – Having pinnules.
  • Pinnules – Generally slender, unbifurcated, uniserial branchlet of arm, typically borne on alternate sides of successive brachials (any plate above the radials or biradials) except hypozygals and axillaries.
  • Plate – Tabular ossicle (calcified segment of skeleton), mostly having articular facets at edges.
  • Primiaxillary – Axillary plate of proximal brachitaxis.
  • Primibrachs – Plates of proximal brachitaxis; may axillary or non-axillary and fixed or free.
  • Quartibrachs - Any ray plates of fourth brachitaxis.
  • Radial - Undivided proximal plate or any ray, may be simple or compound and bearing proximal anal plate.
  • Ramule – Bifurcating or nonbifurcating minor branch of arms, differing from pinnule in less regular occurrence and in some crinoids by presence of pinnules on it.
  • Ray - Radial plate, together with all the structures borne by it.
  • Secundibrachs – Any ray plates of second brachitaxis.
  • Stem – Stalklike attachment of many crinoids; column.
  • Tegmen – Adoral part of theca above origin of free arms or occupying space between them; may be calcified or not; may bear anal vent, pyramid, sac, or tube.
  • Tertibrachs – Any ray plates of third brachitaxis.
  • Theca - Crinoid skeleton exclusive of pelma (column and holdfast) and free arms.
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Image modified from Regnell, 1966

The following image is provided to simply give an idea of basic external characteristics of Edrioasteriods.


          Edrioasteriod Morphology (modified from Regnell, 1966)                
Edrioasteriod (Echinozoan) morphology based on Edrioaster bigsbyi


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Images from Sprinkle and Kier, 1987

The images below illustrate the external characteristics of Stylophora (left thumbnail) and Homoiostele (right thumbnail) Homalozoans.


      Stylophora Morphology (from Sprinkle and Kier, 1987)   Homoiostele Morphology (modified from Sprinkle and Kier, 1987)    
Morphology of Stylophora Based on Phyllocystis
Morphology of Homoiostele Based on Iowacystis (A, upper side, B, lower side, C, section through the theca)


Definitions of terms taken from Ubaghs, 1967


  • Placocystitid plate – Special supracentral located on mid-line of theca and generally surrounded by its two neighbors, distinguished by its smooth surface and its rounded (rather than polygonal) outline.
  • Carapace – Upper face of theca.
  • Aulacophores – Single jointed exothecal appendage, here interpreted as brachial process carrying ambulacrum (structure in which food is conveyed to the mouth).
  • Theca – Body skeleton exclusive of aulacophore.



  • Stele – Relatively slender and tapering peduncular prolongation of the body.
  • Theca – Body skeleton exclusive of stele.


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Images taken from Keen, 1969 and Cox, 1969

The images below display basic features of bivalve morphology, including internal and external characteristics.


      Internal Morphology (from Keen, 1969)   External morphology (from Cox, 1969)    
Internal Morphology (1a right valve interior view, 1b left valve exterior view)
External Morphology (1a right valve interior view, 1b dorsal view, 1c left valve external view)


Definitions of terms taken from Cox, 1969

  • Adductor muscle – Muscle, commonly one of 2, connecting 2 valves of shell, tending to draw them together.
  • Anisomyarian musculature – With one adductor muscle (anterior) much reduced or absent.
  • Auricle – Earlike extension of dorsal region of shell, commonly separated from body of shell by notch or sinus.
  • Auriculate – With auricles.
  • Beak – Noselike angle, located along or above hinge margin, marking point where growth of shell started.
  • Byssal notch – Indentation below anterior auricle of right valve in many Pectinacea (scallops) for passage of byssus (a bundle of hairlike strands used for attachment) or protrusion of foot (a muscular structure for burrowing and locomotion).
  • Byssally attached – Temporary attachment made by bivalves to extraneous objects by means of a bundle of hairlike strands.
  • Carina – Prominent keel-like ridge.
  • Cardinal area – Flat or slightly concave, commonly triangular surface extending between beak and hinge margin in many bivalves, and partly or wholly occupied by ligament.
  • Cardinal teeth – Hinge teeth situated close to the beak.
  • Concentric – With direction coinciding with that of growth lines. (By no means concentric in literal and geometrical sense of term).
  • Costellae – Rather narrow linear elevations of the surface of shell.
  • Duplivincular ligament – Type of ligament with lamellar component repeated as series of bands, each with its 2 edges inserted in narrow grooves in cardinal areas of respective valves.
  • Equilateral - With parts of shell anterior and posterior to beaks equal in length or almost so.
  • Equivalved – With 2 valves of same shape and size.
  • Gape – Localized opening remaining between margins of shell when valves are drawn together by adductor muscles.
  • Growth line – Line on the surface of shell, one of usually irregularly arranged series, marking position of margin at some stage in growth.
  • Hinge – Collective term for structures of dorsal region which function during opening and closing of valves.
  • Hinge tooth - Shelly structure (usually one of a series) adjacent to dorsal margin and recieved in socket in opposit valve; hinge teeth serve to hold valves in position when closed.
  • Inequilateral – With parts of shell anterior and posterior to beaks differing appreciably in length.
  • Ligament – Horny elastic structure or structures joining 2 valves of shell dorsally and acting as spring causing them to open when adductor muscles relax.
  • Lunule – Depression, commonly cordate in shape, present anterior to beaks in many bivalves.
  • Mantle – Integment that surrounds vital organs of mollusk and secretes shell.
  • Modioliform – Shaped like shell of Modiolus; differing from mytiliform in that beaks are not quite terminal and anteroventral region forms slight bulge.
  • Muscle scar – Impression on anterior of shell marking former place of attachment of a muscle.
  • Opisthocline – Sloping (from lower end) in posterior direction (term applied to hinge teeth and, in some genera, to body of shell).
  • Opisthogyrate – Curved so that beaks point in posterior direction.
  • Pallial line - Line or narrow band on interior of valve close to margin, marking the line of attachement of marginal muscles of the mantle (integment that surrounds vital organs).
  • Pallial sinus – Line or narrow band on interior of valve close to margin, marking line of attachment of marginal muscle of mantle.
  • Posterior lateral tooth - Lateral tooth situated posteriorly to beaks and (in heterodonts which have distinctly differentiated cardinal and lateral teeth) posteriorly to ligament.
  • Prosogyrate – Curved so that beaks point in anterior direction.
  • Radial – Direction of growth outward from beak at any point on surface of shell, commonly indicated by direction of costa (a broad prominent elevation) or other element of ornament.
  • Resilifer – Recess or process for attachment of internal ligament.
  • Siphonate – Having a siphon (a tubelike extension of mantle for passage of inhalant or exhalent current).
  • Socket – Recess for reception of hinge tooth of opposite valve.
  • Solemyiform – Recess for reception of hinge tooth of opposite valve.
  • Sulcus – Lateral depression of the surface of shell.
  • Truncate – With curvature of outline interrupted by straight cut.
  • Valve – One of the calcareous structures (2 in most bivalves) of which shell consists.
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Images taken from Teichert, 1964

The labeled images below display some morphological attributes of cephalopods, specifically of the nautiloids.


Orthoconic nautiloid (from Teichert, 1964)   Cephalopod Orientation (from Teichert, 1964)  
Diagram of an Orthoconic Nautiloid. (The septa are spaced widely in the anterior of the phragmacone to show thier shape.)
Sketch showing the possible orientations of cephalopod shells. ( A "conventional", B "hypothetical")
      Nautiloid shell shapes (From Teichert, 1964)              
      Various nautiloid shell shapes.              


Definitions of terms taken from Teichert, 1964

  • Actinosiphonate – Descriptive of endosiphuncular deposits consisting of radially arranged longitudinal lamellae.
  • Annulation – Ringlike expansion of conch, either transverse or slightly oblique to longitudinal axis of conch.
  • Annulosiphonate deposits – Descriptive of endosiphuncular deposits having shape of rings formed at septal foramen and attached to inside of septal neck.
  • Aperture – Opening of shell through which head-foot protrudes.
  • Body chamber – Large undivided, anterior space in conch at open aperture, occupied by animal’s body.
  • Brevicone – Short rapidly expanding conch.
  • Camera (pl., camerae) – Space enclosed between two adjacent septa but not including siphuncle.
  • Cameral deposits – Calcareous deposits secreted against original walls of camerae during life of animal.
  • Ciruculus – Cameral deposit on concave surface of cyrtochoanitic septal neck.
  • Conch – All hard calcareous parts secreted by nautiloid animal including external shell, septa, and siphuncle, but not cameral deposits and any structures within the siphuncle (syn., test).
  • Conchal furrow – Shallow groove on inside of conch wall, located mid-ventrally.
  • Connecting rings – Partly calcareous, partly conchiolinous, tubular membrane, that connects septal neck of ellipochoanitic (having relatively short retrochoanitic septal necks which do not reach as far as preceeding septum) conchs with septum immediately behind it; vestiges of it are also found in holochoanitic forms.
  • Cyrtochoanitic – Descriptive of comparatively short, retrochoanitic septal necks which are curved so as to be concave outward.
  • Cyrtocone – Curved conch that completes less than one whorl.
  • Dorsomyarian – Descriptive term applied to nautiloids in which retractor muscles of head-foot are attached to shell along interior areas of body chamber adjacent to, or coincident with, its dorsal mid-line.
  • Dorsum – Side of animal or conch opposite venter. Among fossil nautiloids, if position of hyponomic sinus (and therefore venter) cannot be established, the term is loosely applied (1) in coiled forms to the concave side of a whorl, (2) in straight or curved conchs with eccentric siphuncle to the side farthest removed from the siphuncle.
  • Endocone– One of series of calcareous cones formed in the posterior or adapical portion of siphuncle mainly of endocerid and discosorid conchs.
  • Endosiphuncle – Space within ectosiphuncle (walls of the siphuncle) including all organic tissues and calcareous structures.
  • Endosiphuncular tube – (1) Circular tube formed by adapical thickening of lateral walls of endosiphocoleon (flattened tube or balde surrounding endosiphuncluar tube) in some Encocerida; (2) simple tubes connecting apices of endocones where no differentiation coleon and tube is possible.
  • Episeptal deposits – Cameral deposits on concave (or adapertural) side of septum.
  • Exogastric – Descriptive of conchs curved or coiled so that venter is on or near outer convex side.
  • Gibbous - A prominent swelling to one side.
  • Growth lines – Markings on surface of conch that denote periodic increases in size and hence former positions of aperture.
  • Gyroconic – Loosley coiled conch in successive whorls are not in contact with each other.
  • Holochoanitic – Descriptive of retrochoanitic septal necks that extend backward through length of one camera.
  • Hyposeptal deposits – Cameral deposits on convex (adapical) side of septum.
  • Hyponomic sinus – Large concave sinus in middle of aperture, marking location of hyponome, invariably ventral.
  • Lirae – Parallel fine ridges or raised lines on surface of conch, transverse or longitudinal, separated by striae. [Restricted to features not easily discernible to the naked eye.]
  • Mural deposit – Cameral deposits along mural parts of septa (the parts of the septum attached to wall of conch).
  • Nummuloidal – Having siphuncular segments that are strongly inflated between septa, mostly in Actinocerida.
  • Orthocone – Straight conch.
  • Othochoanitic - Descriptive of retrochoanitic septal necks which are straight, cylindrical, and extend only a short way to preceeding septum.
  • Phragmocone – Chambered portion of conch.
  • Retrochoanitic – Descriptive of septal necks which are directed backward.
  • Septal foramen – Opening in septum allowing passage of siphuncular cord.
  • Septal neck – Portion of septum which is bent adapically (or backward) around septal foramen.
  • Septum – Partitions dividing phragmocone into camerae and attached to inside of wall of conch.
  • Sinus – Any part of transverse feature (apertural margin, ribs, growth-lines) concave toward aperture.
  • Siphuncle – Long slender or thick tube extending through all camerae to apex and consisting of soft and shelly parts, including septal necks, connecting rings, calcareous deposits, and siphuncular cord (adj., siphuncular).
  • Siphuncular - See siphuncle.
  • Striae – Parallel, small to minute grooves or channels on surface of conch, either transverse or longitudinal, separated by lirae. [Restricted to features not easily discernible to the naked eye.]
  • Suborthochoanitic – Descriptive of retrochoanitic septal necks that are short and straight, with slightly outwardly inclined tips but with no measurable brim.
  • Suture – Line of junction of free part of septum and inner side of phragmocone wall.
  • Test – See conch.
  • Torticone – Conch coiled in helicoidal spire, as in gastropods.
  • Venter – Underside of organism and of conch, distinguished generally by hyponomic sinus and often by conchal furrow.
  • Whorl – Complete turn of coiled conch (syn., volution).
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Image from Peel in Boardman, Cheetham, and Rowell, 1987

Labeled on the image below are many of the important morphologic traits of a gastropod shell. Note: Some of the features of these shells are not present on the Ordovician gastropods represented in the Trenton Group.


          Gastropod Hard Parts Anatomy (from Peel in Boardman, Cheetham, and Rowell, 1987)              
Gastropod hard part anatomy.


Definitions of terms from Cox, 1960

  • Abapical – Away from shell apex toward base along axis or slightly oblique to it.
  • Adapical – Toward shell apex along axis or slightly oblique to it.
  • Aperture – Opening at last formed margin of shell, providing outlet for the head-fott mass.
  • Base – In conispiral shells ( with spire projecting as cone), part of surface lying on abapical side of periphery of last whorl or of a carina or angulation that forms an obvious lower boundary on the side of whorl.
  • Cancellate – Having ornament of intersecting spiral and transverse threads or cords.
  • Carina – Prominent spiral ridge or keel.
  • Columella – Solid or hollow pillar surrounding axis of a coiled shell, formed by adaxial walls of whorls.
  • Cord – Round-topped moderately coarse spiral or transverse linear elevation on shell surface.
  • Dextral – Right handed; term originally applied to any shell with aperture on observer's right when shell apex is directed upward, or with apparent clockwise coiling when viewed from above apex, but in fact definition depends on features of soft anatomy. A dextral gastropod has genitalia on the right side of the head-foot mass or pallial cavity and the shell of such an animal commonly has the aperature on the right when viewed with the apex uppermost.
  • Disjunct – Condition of whorls when out of contact.
  • Dorsum – Dorsal side; side opposite the venter.
  • Growth lines – Collabrally disposed surface markings of shell, generally not prominent as to relief, that denote former positions of outer lip.
  • Hyperstrophic – Dextral anatomically, with genitalia on the right, but shell appears as if the anatomy would be on the left side.
  • Inductura – Smooth shelly layer secreted by general surface of mantle, commonly extending from inner side of aperture over parietal region, columellar lip, and (in some genera) part or all of shell exterior.
  • Inner lip – Adaxial margin of aperture extending from foot of columella to suture and consisting of columellar and parietal lips.
  • Involute – With the last whorl enveloping earlier ones so that height (or “width” in shells like bellerophonts) of aperture corresponds to that of shell; early whorls more or less visible in umbilici.
  • Lamellae – Thin plates.
  • Lamellose – Having lamellae.
  • Lanceolate – Shaped like a lance-head (i.e., sharply pointed at one end), broader at the other.
  • Lunulae – Crescentic linear ridge on selenizone, concave toward aperture.
  • Nucleus – Earliest-formed part of shell.
  • Orthostrophic – Coiled in normal manner, not hyperstrophic.
  • Outer lip – Abaxial margin of aperture extending from suture to foot of columella.
  • Parietal region – Basal surface of helicocone just within and just without aperture; the redundant expression “parietal wall” should not be used, for “parietal” signifies pertaining to wall.
  • Peristome – Margin of aperture.
  • Phaneromphalous – With completely open umbilicus; may be wide, narrow, or very minute.
  • Ramp – Abapically inclined flattened band on shell surface, which in some shells forms the adapical part of whorls, limited abaxially by ridge or angulation.
  • Revolving – Passing spirally around whorls.
  • Selenizone – Spiral band of crescentic growth lines or threads (lunulae) generated by a narrow notch or slit and characteristic of dibranchiate gastropods.
  • Septum – Transverse plate secreted within early-formed whorls of some shells for closing them off.
  • Shoulder – Angulation of whorl forming abaxial edge of sutural ramp or shelf.
  • Sinus – Curved re-entrant of apertural margin or of growth lines.
  • Slit – Parallel-sided re-entrant of outer lip ranging from shallow incision to deep fissure as half a whorl in extent.
  • Spire – Adapical visible part of all whorls except last.
  • Sutural shelf – Horizontal flattened band, which in some shells adjoins adapical suture of whorls.
  • Suture – Continuous line on shell surface where whorls adjoin.
  • Thread – Fine linear surface elevation.
  • Trochiform – With flat-sided conical, not highly acute spire and rather flat base, like shell of Trochus.
  • Turbiniform – With broadly conical spire and convex base, as in shell of Turbo .
  • Umbilicus – Cavity or depression formed around shell axis between faces of adaxial walls of whorls where these do not coalesce to form a solid columella; in conispiral shells (except hyperstrophic ones) its opening is at base of shell but involute shells may have two umbilici.
  • Venter – Ventral side; the underside of the animal.
  • Ventricose – Inflated or swollen, especially to one side.
  • Volution – Any complete coil of helicocone (a distally expanding coiled tube that forms most gastropod shells).
  • Whorl – (1) Any complete coil of helicocone (a distally expanding coiled tube that forms most gastropod shells); (2) exposed surface of any complete coil of helicocone.
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Image modified from Knight and Yochelson, 1960


        Dorsal and Ventral Morphology (after Knight and Yochelson, 1960)        
A sketch of the Recent monoplacophoran Neopilina showing (A) the ventral side with labeled body parts and (B) the dorsal side showing details of the shell.





Image modified from Moore and Harrington, 1967

The image below shows the external characteristics of conularids.


          Conularids (modified from Moore and Harrington, 1956)                
Conularid Features
a, b, corner and facial views, c, transverse section
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Images from Berry in Boardman, Cheetham, and Rowell, 1987

The images below show examples of the principle soft part anatomy of graptolites and the hard exterior morphology (which is commonly preserved).


      Soft Part Anatomy (from Berry in Boardman, Cheetham, and Rowell, 1987)   Morphological Forms (from Berry in Boardman, Cheetham, and Rowell, 1987)    
Soft Part Anatomy of Graptolite Zooid
Variations in Form
          Hard Part Morphology (from Berry in Boardman, Cheetham, and Rowell, 1987)                
Hard Part Morphology of Graptolites


Definitions of terms from Bulman, 1970

  • Adnate thecae – Thecae that have adhered together during growth.
  • Amplexograptid theca – Strongly geniculate theca with deep and long, rounded apertural excavations, generally with infragenicular selvage and typically with low rounded apertural lappets.
  • Apertural spine – Projection originating on margin of aperture; commonly single, less commonly paired.
  • Aseptate – Biserial rhabdosome lacking median septum (a partition in biserial graptoloids separating two series of thecae).
  • Axillary angle – Angle at the base of V-shaped bifurcation of dichotomously branched rhabdosomes, and especially bifurcation of dicranograptids.
  • Biserial - Scandent (stipes growing erect and enclosing or including the nema/virgula) graptoloid rhabdosome with two series of thecae enclosing nema.
  • Branch – See stipe.
  • Climacograpid theca - strongly geniculate theca with straight or slightly convex supragenicular wall parallel to axis of rhabdosome and relatively short (narrow) apertural excavation.
  • Dicalycal theca – Graptoloid theca giving rise to two buds ( c.f. normal asexual reproduction in which single bud is produced by each zooid.
  • Everted – Plane of aperture facing outward (cf. introverted, retroverted)
  • Geniculum – Angular bend in direction of growth of graptoloid theca, especially climacograptid or lasiograptid; hence supragenicular, infragenicular.
  • Geniculate - Has an angular bend in direction of growth of theca.
  • Introverted – Plane of aperture facing inward (dorsally), resulting from excessive growth of ventral wall of theca usually accompanied by sigmoidal curvature of thecal axis.
  • Isolation – Separation of distal (metathecal) portions of thecae from stipe.
  • Mesial – Middle portion of free ventral wall (supragenicular wall) of theca; hence mesial spine.
  • Metatheca - distal portion of graptoloid theca, morphologically equivalent to autotheca of dendroids, etc.
  • Nema – Threadlike extension of apex of prosicula, extending embryonic nema prosiculae ; probably solid in adult rhabdosomes. May have served for attachment or may terminate in disc of attachment or vanelike “float” structures. Known as virgula in scandent (stipes growing erect and enclosing or including the nema/virgula) graptoloids.
  • Orthograptid theca - Straight, parallel-sided, tubular theca of biserial graptoloid.
  • Periderm – Horny substance of scleroproteic composition forming skeleton of Graptolithina, comprising inner (fusellar) layer with growth bands and growth lines and outer (cortical) layer of finely laminated tissue.
  • Prosicula – Proximal, initially formed part of sicula, apparently selected as single conical unit with faintly marked spiral thread; at later stage longitudinal fibers are added.
  • Rhabdosome – Sclerotized exoskeleton of entire graptolithine colony.
  • Selvage – Thickened margin, especially of aperture.
  • Septate - Rhabdosome having a median septum.
  • Sicula – Skeleton of initial zooid of colony, comprising conical prosicula and tubular distal metasicula.
  • Stipe – One branch of a branched rhabdosome or entire colony of unbranched rhabdosome.
  • Subapertural spine – See apertural spine.
  • Th 11, 12, 21, etc. – Th 11 is the technical term for the first formed theca in the branching series of thecae that make up many graptolite exoskeletons. The following thecae are consecutively numbered in the same manner.
  • Theca (pl., thecae) – Sclerotized tube or cup (other than sicula) enclosing any zooid of rhabdosome.
  • Uniserial - Rhabdosome or stipe of graptoloid consisting of single row of thecae only.
  • Virgella – Spine developed during growth of metasicula (distal portion of sicula), embedded in sicular wall and projecting freely from its apertural margin.
  • Zooid – Soft-bodied individual inhabiting theca.
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Images from Adrain, 1992

The sketch below illustrates two different kinds of Machaeridian body plans.


          Machaeridian Morphology (from Adrain, 1992)                
Machaeridian Reconstruction


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Contact Curatorial Associate

© 2004 President and Fellows of Harvard College

Specimen of an unidentified ostracode, MCZ 145810 Specimen of an unidentified ostracode, MCZ 145801 Specimen of an unidentified ostracode, MCZ 145800 Specimen of Hypodicranotus striatulus, MCZ 100986 Specimen of Ceraurus pleurexanthemus, MCZ 100811 Specimen of Isotelus gigas, MCZ 145649 Specimen of Zygospira modesta, MCZ 145777 Specimen of Lingula quadrata, MCZ 145759 Specimen of Rafinesquina trentonensis, MCZ 145769 Specimen of Platystrophia amoena, MCZ 144884 Specimen of Salteraster medusa, MCZ 108069 Specimens of Macroporaster matutinus, MCZ 108055, MCZ 146701, and MCZ 146702 Specimens of Cheirocystis anatiformis, MCZ 145749 and MCZ 145750 Specimen of Cincinnaticrinus varibrachialus, MCZ 136876 Specimen of Ectenocrinus simplex, MCZ 133422 Specimen of Edrioaster? sp., MCZ 125601 Specimen of Atelocystites cf. A. huxleyi, MCZ 128266 Specimen of Palaeoneilo sp., MCZ 143864 Specimen of Ambonychiopsis sp., MCZ 145737 Specimen of Cuneamya miamiensis, MCZ 113853 Specimen of Leurocycloceras sp., MCZ 130123 Specimen of Parakionoceras sp., MCZ 131051 Specimen of Lechritrochoceras sp., MCZ 135388 Specimen of Holopea symmetrica, MCZ 145381 Specimen of Sinuites cancellatus, MCZ 145409 Specimen of Globonema salteri cf., MCZ 145379 Specimen of Cyclonema bilix aff., MCZ 145375 Specimen of ?Archinacella arca aff., MCZ 145367 Specimen of Conularia quadrata, MCZ 114219 Specimen of Orthograptus quadrimucronatus, MCZ 146495 Specimen of Rectograptus amplexicaulis, MCZ 145787 Specimen of Plumulites sp., MCZ 145745