182 PALEONTOLOGY OF ILLINOIS. 



So that the only distinction left would be the presence of but two primary 

 radiala to each ray, while Prof. Hall has referred some species possessing this 

 latter character to Poteriocrinus proper. 



The group Coeliocrinus, so far as known, seems to differ from Scaplviocrinus 

 only in having the proboscis venticose or balloon-shaped above, instead of cylin- 

 drical, while it differs from the typical forms of Poteriocrinus both in that 

 character and the presence of but two primary radial pieces in each ray. It is 

 possible, however, that when the position of the opening of the summit is 

 determined in Coeliocrinus, that it may be found to differ from that of Poterio- 

 crinus proper. Dr. White thinks it must be situated near the base of the 

 proboscis, as he could find no traces of it in specimens where all the upper parts 

 of the proboscis were well preserved. 



In accordance with the views of de Koninck and Lehon and other authors, 

 we regard the genus Poteriocrinus as dating back at least to the Upper Silurian 

 Epoch. Two English species have been referred to it from that horizon by 

 de Koninck and Lehon ( Gyatlwcrinus goniodactylus, and C. capillaris, Phillips); 

 while Homocrinus scoparius of Hall (Palseont., N. Y., iii, pi. 1), certainly agrees 

 with the typical forms of the genus far more nearly than the type of Scaphio- 

 crinus, which Prof. Hall, and all others, place as a section of Poteriocrinus. In 

 fact it seems to be in every respect a typical Poteriocrinus — whether we regard 

 its general physiognomy, the form and structure of its proboscis, its rays or 

 body, with the single slight exception of having the lower one of its two anal 

 pieces, below the top of the first radials, isolated from those above the one con- 

 necting with its right superior sloping side. But if this very slight peculiarity 

 of the lowest anal plate excludes this species from Poteriocrinus, how can that 

 group include as one of its sections Scaphiocrinus, in the type of which this lowest 

 anal plate is entirely wanting, and the whole physiognomy more widely different? 

 Again, if this arrangement of the lowest anal plate excludes it from Poterio- 

 crinus, how can it, upon such a basis of classification, be referred to Homocri- 

 nus?, the type of which (ZZ~. parvus) presents the marked difference of having 

 the lowest anal piece resting directly down upon the basal pieces, to say nothing 

 of the wide differences in the structure of the arms. 



The genus Poteriocrinus also occurs in the Devonian rocks, both in this 

 country and Europe. P. minutus, Roemer, is a European example; while 

 Prof. Hall has described several species from the Hamilton division of the 

 Devonian of this country. The so-called Cyathocrinus ornatissimus, Hall, from 

 the Portage group of N. Y., is probably also a Poteriocrinus, judging from the 

 figure. , 



It was during the Carboniferous epoch, however, that this genus undoubtedly 

 attained its greatest development, and we have no evidence of its existence 

 after the close of that period of our earth's history. 



