306 PROCEEDINGS OF THE ACADEMY OF [1819. 



Sphserocrinus Roemer is founded upon Cyathocrinus geometri- 

 cus Goldfnss, and it has all the characters of Cyathocrinus, not 

 only in the construction of the calyx, but also of the vault, and 

 we find a contraction of the body in the arm regions in the major- 

 ity of its species. 



Palseocrinus Billings is not distinct from Cyathocrinus. The 

 construction of the calyx is identical, and the five calycinal grooves 

 radiating from the centre of the abdominal surface, which, accord- 

 ing to Billings, form the principal distinction, are identical with the 

 ambulacral grooves passing here as there over the sutures of the 

 oral plates. Cyathocrinus, especially in its Lower Silurian form, 

 and when deprived of the arms, bears the closest relation to some 

 forms of the Blastoids on one side, and the Pentacrinoid larva 

 on the other. We propose the following: 



Revised Generic Diagnosis: General form of the body with 

 arms, elongate ; calyx cyathiform, usually with convex sides, in- 

 curving toward the upper margin, and, therefore, subglobose in 

 general outline ; symmetry bilateral. 



Underbasals five, moderately large, of equal size, either spread 

 out horizontally or with a slight upward curvature. Basals large, 

 regularly alternating with the radials, with acute upper angles, ex- 

 cept the posterior one which is truncate for the support of a single 

 anal plate. Radials one by five, as large or larger than the basals, 

 incurving toward the vault. Articulating facet rarely occupying 

 more than one-half the width of the plate often less than one-third 

 and one fourth to one-third its height, It is on the exterior of the 

 plate, circular or elliptic in form, and provided either with a small 

 perforation, or more frequentlj" with a deep notch which connects 

 with the arm groove. The brachials are similar in construction 

 to the arm joints, and their number, as a rule, is very irregular, 

 varying sometimes from two to seven in the same specimen, and 

 even in arms of the same relative position, scarcely two rays 

 having an equal number. 



Arms long, branching, each branch bifurcating several times, 

 and as they gradually taper toward the tips they become very 

 slender above. The arm joints, as well as the brachials in Silu- 

 rian species, are shorter, and comparatively heavier than in spe- 

 cies from the Subcarboniferous. In the former they are about as 

 wide as high, in their later representatives generally three or four 

 times higher than wide. Both, however, agree in the absence of 



