242 PALAEONTOLOGY OF ILLINOIS. 



Genus ONYCHOCRINUS, Lyon and Casseday. 



Si/non. — Onychocrinus, Lyon and Casseday, 1859. Am. Journal Sci (2), xxix, p. It. 



Forbesiocrinus (sp.), Hall, 1858; Iowa Report. Meek and Worthen (sp.), 

 1860; Proceed. Acad. Nat. Sci., Philad.; (not deKoninck and Lehon.) 



Although for some time past inclined to regard the type for which the name 

 Onychocrinus was proposed, probably in no respect distinguishable from Forbes- 

 iocrinus, a careful study and comparison of a number of specimens of these 

 groups has led us to believe them most probably generically distinct. At any 

 rate, they certainly differ upon more constant and more clearly defined charac- 

 ters than those separating Forbesiocrinus from Taxocrinus, which we have else- 

 where shown,* blend into each other by such imperceptible gradations, that we 

 we do not believe they can be separated more than subgenerically. 



In the nature of the column, the number and arrangement of the basal, sub- 

 radial and primary radial pieces, Onychocrinus agrees exactly with Forbesiocrinus, 

 while in other respects the species of the former, particularly the typical species, 

 differ from those of the latter in some points of structure that impart a peculiar 

 physiognomy to the whole fossil, quite obvious even to the most careless observer, 

 in examining entire specimens. In the first place, they differ from Forbesio- 

 crinus in having the body more depressed, and the rays, from their origin, more 

 diverging, or in some cases extending out horizontally on the same plane with 

 the base; while in these extreme cases the long rays, which are free in to the 

 second radials inclusive, bear the small arms in clusters at their extremities, 

 remote from the body. In short, if we could open and spread out the rays of 

 a Forbesiocrinus upon a plane as far in as to the second radial pieces, and then 

 divide each of the interradial series of plates, and fold them up so as to cover 

 the vault, and apparently to some extent the free rays, somewhat in the manner 

 suggested by Prof. Agassiz for the ideal conversion of an Echinoid into a star- 

 fish, we would have a form very like the extreme Onychocrinus. Still there 

 would be this difference, however, that in Onychocrinus the anal series of plates 

 is almost entirely wanting, and the anal area open and so deeply excavated as 

 often to completely destroy the symmetry of the body. That is, instead of 

 having the anal area occupied by as many pieces as, or a greater number than, 

 occupy the interradial areas, as in Forbesiocrinus, we find the whole anal series 

 in Onychocrinus, whatever may be the number of interradials, reduced to a few 

 very small pieces, ranged in a direct line, one upon another, and resting with 

 the first or lowest one in a small sinus in the middle of the upper side of ihe 

 large odd subradial. much as the arms of Platycrinus rest upon tbe first radials, 



* Proceedings Academy Natural Science, Philadelphia, August, 1865, p. 138. 



