HETEROCRINIDAE 
III 
EUCHEIROCRINUS Meek and Wortheii 
Plate 2g 
Cheirocrinus Hall (not Eichwald), 13th Rep. New York St. Cab. Nat. Hist., i860, p. 122. — Eticheirocrinus 
Meek and Worthen, Proc. Acad. Nat. Sci. Phil., 1869, p. 73; Geol. Surv. Illinois, 5, 1873, pp. 443, 
502. — Bather, Crinoidea of Gotland, 1893, pp. 56, 61, fig. i^b. — Zittel-Eastman, Textb. Pal., 2d ed., 
1913, p. 213. — Bassler, Bibliographic Index, Bull. 95, U. S. Nat. Mus., 1915, p. 508 . — Proclivocrinus 
Ringueberg, Ann. New York Acad. Sci., 4, 1889, p. 396 (sep., p. 9). 
Arm-bearing rays 3, the r. ant. arm having entirely disappeared. Stem 
shifted back to median position in plane of anal tube, and perfect bilateral sym- 
metry restored. Median arm simple or branching; segments of its 1 . ant. com- 
pound radial usually narrowdy connected, inferradial widening downward. 
Lateral arms symmetric, one on a side; unequally dichotomous, dividing into 
two branches, which bifurcate once or twice again; r. post, and r. ant. super- 
radials fused to form subanal piece supporting anal tube, and separating the 
respective inferradials from one another and from anal x. Lateral primibrachs 
usually 2, the axillary with unequal faces. Basals 4, as in Creinacrinus, with 
suture between left BB sometimes obscure. 
Genotype. Cheirocrinus chrysalis Hall. 
Distribution. Silurian ; America, England. 
This Silurian form has thus lost the fourth arm, and recovered the symmetric position 
of the stem, but has not yet acquired the doubly heterotomous mode of branching of the two 
lateral rays, which marks the axil-arm system — although the unecjual-faced primibrach prob- 
ably forms the first step towards it. The arm structure, although in the main rather simple, 
is subject to considerable variation. In the typical form the lateral arm divides on the axillary- 
second primibrach, and branches again. The median arm typically is simple, or only diwd'es 
high up, while in E. Ontario and the English species E. anglictis it branches near the calyx and 
at least twice above. In E. chrysalis this arm is relatively massive, and in the other two species 
its robustness is compensated by the lower and more numerous bifurcations. Thus the arms 
in this genus may be considered to be in a state of flux, leading toward the axil-arm plan. In 
the connection of the segments of 1. ant. radial it represents a rather intermediate stage between 
the Ordovician Cremacrinus and the Devonian and Carboniferous Halysiocrinus, the tendency 
being for the inferradial to become narrower upward. The base remains substantially as in 
Cremacrinus, the unfused left BB together being small, touching the stem, and having the same 
characteristic curved outline, with a tendency to coalesce, which became a fixed character in 
the succeeding genera. 
’ The genus as now understood has a considerable distribution, from New York, Canada, 
Indiana and Tennessee in America to Dudley in England. It is essentially Silurian, but it 
may be that Calceocrinus barrandei Walcott, from the Trenton, should be referred to it. 
