294 PROCEEDINGS OP THE ACADEMY OP [1879. 



for a subgenus. It seems that he was in doubt whether Iocrinus 

 should be placed with Helerocrinus or Poteriocrinus, as he sup- 

 posed these two genera to be closely related. In this Hall is cer- 

 tainly in error, since a close comparison proves them to be very 

 distinct. Neither can we conceive how such forms as Iocrinus 

 crassus can be referred to Heterocrinus. The two differ essen- 

 tially in the anal arrangement, and in the form and construction 

 of the radial plates, which are perfectly symmetrical and simple 

 in the former, but irregular and compound in the latter. Such 

 characters have heretofore always been considered of generic im- 

 portance, and we accordingly adopt Prof. Hall's name but in a full 

 generic sense, and propose for the genus Iocrinus the following: 



Generic Diagnosis. General appearance somewhat similar to 

 Pentacrinus; comparatively larger than Heterocrinus; arms 

 longer and more frequently bifurcating; calyx more broadly 

 spreading, and perfectby symmetrical up to the top of the radials, 

 giving the form of a short, inverted, pentagonal pyramid with 

 the five sides deeply concave. 



Underbasals undeveloped. Basals small, pentagonal. Radials 

 comparatively large, strong, all pentagonal, and of the same 

 height ; their upper margins truncated for nearly their entire 

 breadth for the junction of the succeeding pieces. Brachials 

 three to four in each ray, the upper one axillary, and supporting 

 the first free divisions of the arms. In the right posterior ray 

 there is interposed between the true brachials and radial plate a 

 pentagonal bifurcating piece, which is evidently free and mova- 

 ble like the brachials, and of the same width. This peculiar plate, 

 which is truly radial, supports on its right sloping side the usual 

 number of brachials, and on the left a row of*quadrangular plates, 

 vertically arranged, extending to the tips of the arms, and form- 

 ing the posterior wall of a large ventral tube. In external ap- 

 pearance these plates resemble the brachials and arm plates, only 

 they are somewhat higher and not quite as wide; they are gib- 

 bous, and form an elevated ridge, which causes this appendage to 

 resemble an arm or a branch of the ray, and so it was considered 

 by Hall in his description of Heterocr. polyxo. Both sides of the 

 mesial ridge are indented to accommodate other plates, of which 

 there are two to each median plate, one abutting against the mid- 

 dle part, and the other opposite the suture. These lateral plates 

 are delicate, three or four times wider than high, and, like the other, 



