PARACRINOIDEA- Description from Parsley and Mintz, 1975. Ecology from Guensburg, 1991.

• Amygdalocystites sp. Billings, 1854

RHOMBIFERA- Description by J. C. Brower

• Cheirocystis anatiformis (Hall, 1847)

Amygdalocystites sp. Billings, 1854

Amygdalocystites Billings, 1854, p. 270-271, fig. 4-9; Billings, 1856, p. 288-290; Billings, 1858, p. 63-65, pl. 6, fig. 1a-1e, 2a-2f, 3a, 6; Zittel, 1879, p. 413; Jaekel, 1900, p. 675, 676; Springer in Zittel, 1913, p. 151; Regnéll, 1945, p. 38, 39; Wilson, 1946, p. 9-11, pl. 1, fig. 1-4; Kesling, 1968, p.269, fig. 1-4; Parsley and Mintz, 1975, p. 44.
Ottawacystites. Wilson, 1946, p. 14, pl. 3, fig. 1.
Amygdalocystis Billings, 1854. Carpenter, 1891, v. 24, p. 27; Haeckel, 1896, p. 106; Bather, 1900, p. 57, fig. 19; Jaekel, 1918, p. 27.

Description and Occurrence: Rare in the Trenton Group. Theca transversely elliptical in cross-section, ovoid to almond-shaped in outline. Thecal plates numerous, hexagonal to octagonal, each plate with prominent prosopon rays radiating from an elevated central boss to the plate corners. Subepistereomal sutural pores well developed, usually adjacent to, or under, prosopon rays. Two uniserial arms recumbent, transverse. Each arm ossicle with free-standing pinnule; biserial covering plates on pinnules and over main food grooves. Periproct on anterior or posterior face. Column slightly tapering, composed of thin ossicles with crenulate sutures; proximal column sharply curved.

Ecology: Amydalocystites was a low-level, attached, rheophylic suspension feeder. Feeding style differed from that of typical stalked echinoderms such as crinoids, which have a circular or parabolic filtration fan. It is possible that the animal could have oriented itself for most effective streamlining with the stem and feeding structures parallel to prevailing currents. Down-current orientation was probably the most advantageous.

Cheirocystis anatiformis (Hall, 1847)

Echino-encrinites anatiformis Hall, 1847, p. 89-90, pl. 29, fig. 4a-4f.
Glyptocystites logani Billings, 1857, p. 282-283; 1858, p. 57-59, pl. 4, fig. 1a-1j; Chapman, 1861, p. 514, fig. 179; 1863, p. 198, fig. 179; Chapman, 1864, p. 170, fig. 179; Grabau and Shimer, 1910, p. 464.
Glyptocystites logani var. gracilis Billings, 1858, p. 59, pl. 4, fig. 2.
Echino-encrinites anatinaformis [sic] Billings, 1858, p. 58.
Glyptocystites anatifomis Miller, 1889, p. 269.
Homocystites anatiformis Whitfield, 1898, p. 26.
Homocystites anatiformis var. logani Whitfield, 1898, p. 26.
Chirocrinus anatiformis Jaekel, 1899, p. 221.
Chirocrinus logani Jaekel, 1899, p. 220.
Glyptocystis logani Springer, 1911, p. 45.
Cheirocrinus logani Bather, 1913, p. 441; Bassler, 1915, p. 213; Bassler and Moodey, 1943, p. 143.
Cheirocrinus logani var. gracilis Bather, 1913, p. 442.
Cheirocrinus logani gracilis Bassler, 1915, p. 213; Bassler and Moodey, 1943, p. 143.
Cheirocrinus anatiformis (Hall, 1847). Bassler, 1915, p. 212; Clark, 1919, p. 10-11, pl. 1, fig. 18; Bassler and Moodey, 1943, p. 142; Hussey, 1952, p. 34; Kesling, 1962, p. 4, fig. 1-4, pl. 1, fig. 1-7, pl. 2, fig. 1-7, pl. 3, fig. 1-5, pl. 4, fig. 1-9.
Cheirocrinus sp. Hussey, 1950, p. 13; 1952, p. 35.
Cheirocrinus plates, Hussey, 1952, p. 36.

Description and Occurrence: Rhombifera Cheirocrinidae. Common at various localities in the Trenton Group. Like some other abundant Trenton forms, C. anatiformis is most frequently associated with members of its own species. One individual from Walcott-Rust Quarry is located on a small slab with Cincinnaticrinus varibrachialus and the only known specimen of Amygdalocystites sp. Disarticulated plates and fragmented calyces of C. anatiformis are moderately common in beds with C. varibrachialus and Ectenocrinus simplex. Theca roughly cylindrical with rounded base and top; plates with prominent and sharp ridges. Pectinirhombs disjunct, often large, with raised centers. Large periproct visible in some specimens. Ambulacra short and restricted to oral area; brachioles biserial, unbranched, bearing four rows of large covering plates. Proximal stem tapering, consists of nodose and non-nodose plates; large axial canal present. Distal stem straight, made up of long and thin columnals with narrow axial canals. Holdfast not known.

Ecology: Although the holdfasts of C. anatiformis have not been seen, columns that are almost complete are common. The maximum known stem length is 37 mm. Hence, an elevation of about 50 mm above the substrate seems reasonable, and the animal clearly fed at a rather low level. C. anatiformis has food grooves that are about 0.7 mm wide which are the largest ones known in any of the Trenton pelmatozoans. Whether or not tubefeet were present in rhombiferans has been debated for many years. Brower (1999) gave a brief review and argued that tubefeet were present in rhombiferans such as Pleurocystites. Clearly, if C. anatiformis had tubefeet like those of crinoids, then it was certainly capable of catching large food particles.