152 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 92 
of the residue. The Pachylocrinidae probably evolved from a 
Devonian stock much like Decadocrinus, or perhaps from Decado- 
crinus itself. 
RHOPOCRINUS, new genus 
Genotype.—Rhopocrinus spinosus, new species. 
Generic diagnosis.— ‘ 
Crown. Medium height, compact. 
Dorsal cup. Broadly turbinate. Plates smooth or with pits at 
angles of plates and low folds normal to the faces. In all known 
species there is a sharply defined vertical groovelike depression 
between adjacent AR. 
IBB. Small, but showing in lateral view. 
BB. Large. 
RR. Large. Articulating facet extends nearly full width of R, linear 
to slightly crescentic. Suture not gaping. 
IBr. Low, stout. Typically two in all rays. 
Arms. Stout in the basal portion, becoming slender distad. Para- 
endotomous—that is, above the J7Aa rami are given off to the 
inside of each half-ray. The intervals between successive rami 
are short, there being as many as four or more rami in series 
above the J/Aw#. This gives the arms a great number of divi- 
sions. The axillaries tend to be spinous, in some cases the spines 
being of considerable length. The brachials are cuneate and 
in some cases do not reach to the far side of the ramus. 
Post IR. RA is large. X is large, the greater part of the plate 
lying above the plane of the radial facets. A7Z is large. As in 
the case of X, but a small part of the plate lies within the dorsal 
cup proper. 
Ventral sac. The ventral sac is formed of large numbers of small 
plates. The sac is wide and extends to three-fourths or more 
of the height of the arms. The distal portion bears a number 
of stout spines. The plates of the sac tend to be nodose, with 
well-defined pits at the angles of the plates. 
Column: The column is circular in section and has a pentalobate 
lumen. The nodals are very prominent and bear cirri to within 
8 cm. of the crown in medium-sized specimens of the type species. 
Characteristic species of the genus.— 
RHOPOCRINUS MUNICIPALIS (Troost), new combination 
Poteriocrinites municipalis Troost, 1849, p. 419, nom. nud. 
Poteriocrinites municipalis Troost, 1850, p. 61, nom. nud. 
Poteriocrinites municipalis Troost, 1909, p. 82, pl. 11, fig. 5. “St. Louis lime- 
stone, Lawrence County, Tennessee.” (Probably Gasper.) 
Cyathocrinus? municipalis (Troost) Woop, 1909, p. 82. 
