TRENTON LIMESTONE. 81 



Genus S C H IZ C R I NU S. 

 [ Greek, (fx'^h '° cleave ; in allusion to the clefl or double interscapular plates.] 



Character. Pelvis composed of five pentagonal plates ; first costals five, joining at their 

 lateral edges ; second costals five, separated by a hexagonal intercostal plate, which rests 

 upon the upper lateral edges of the lower costals ; scapular and arm-plates five, hexagonal, 

 the lower side curved and fitting the concave upper edge of the plates below ; hand-joint 

 double or bipartite, as also the interscapular or interbrachial plates ; hands and fingers 

 regularly bifurcating. 



This genus, in its general features, bears a close resemblance to some species of the 

 Cyathocrinus ; but the greater number of series of plates, and the duplicate character of 

 the interscapular plates, is suflicient to separate it from that genus. It bears considerable 

 resemblance in its characters to the Dimerocrinites of Phillips, presenting nearly the same 

 deviations from Cyathocrinus which that genus does from Actinocrinites. 



 ■'*|, 

 122. 1. SCHIZOCRINUS NODOSUS (n. jjp.). , .^> 



Pi.. XXVII. Figs. I a- p. 



Body cupshaped, obtusely pentagonal, spreading somewhat abruptly from the column ; 

 arms short ; fingers in ten pairs, fimbriated ; column round, composed of joints of unequal 

 thickness and diameter, the larger ones furnished with side-arms ; surface of the joints 

 deeply striated in radii, which give a serrated appearance to the edges of the plates. 



This species is perhaps the most abundant of any in the Trenton limestone, though 

 usually seen only in fragments of columns. The column, as in many other species, varies 

 considerably in appearance at diflferent distances from the body ; which characters require 

 to be considered, in order to recognize the same when found in small fragments. The thin 

 upper plates of the column are about equal in thickness and diameter ; and for some distance 

 below this the column is composed of alternating larger and smaller plates, the larger ones 

 being thicker, and gradually becoming nodulose on their margins. In tracing the column 

 downward, there is a gradual increase in number of the smaller plates between the larger 

 ones ; the second stage being two, the third three, and the fourth four ; and, in the mean 

 time, the larger plates become furnished with side-arms. When the number of intermediate 

 plates between the arm-plates reaches the number of five, the central one shows a tendency 

 to thickening and enlargement, and does, at least in a great number of instances, assume 

 the character of the thicker armed plates, which finally become equally developed with the 

 others. This change in the character of the column at different distances from the body v 



presents a great variety of aspect, and, when examined in fragments, is liable to lead the 

 student astray, by inducing him to refer fragments of the same species to different ones. 



In weathered specimens of this column, the edges present a beautiful serrated appearance, 

 from the elevated striae upon the plates of the column joining together, the elevations of 

 one filling the depression in the other. The side-arms of the larger joints are rarely seen of 

 any considerable length, the bases being all that usually remain of them. 

 [Paleontology.] 11 



