_ 368 Wachsmuth and Springer—Paleocrinoidea. | 
Volborth has established a genus for the Russian form and this 
cannot be disregarded without great inconsistency. That the 
one azygous plate in the Russian species is probably equiva- 
lent to the two in the Canadian, which we do not dispute, does 
not in our opinion alter the case. We have already shown 
(Revision I, pp. 65-75), that the differences among the plates 
of the azygous side in some of these genera are the result of 
modifications from one to another, and we hope to prove 
further on that the plates which constitute the azygous side, 
both special anal plates and adjoining radial, had a common 
- origin in all these genera, and were gradually evolved from a 
simple azygous plate. 
_ There is, however, another good distinction between Hybo- 
crinus and Hoplocrinus, to his, as yet, little attention has 
been paid, although it has been indicated by Wetherby (Cin- 
cin. Journ. Nat. Hist., July, 1880), when he described the 
upper azy. at mcs “as rounded and crenulated at its distal 
extremity.” etherby had discovered in Mercer county, Ky., 
associated with Hybocystites problematicus, in a siliceous lime- 
stone at the upper part of the Trenton group, several well-pre- 
served specimens of Hybocrinus twmidus Billings, which he 
illustrated by several figures. Fig. 2 on Plate V, represents 
the azygous side of a specimen “showing the crenulated and 
convex upper face of the azygous plate,” a structure which had 
not been found in any of the Russian specimens, and which 
nig saan did not possess, if reliance can be placed upon 
“the figures. 
Wetherby states that in Hybocrinus “the form of the (azy- 
gous) plate is sufficient evidence, that it supports a strong ven- 
tral sac,” and he says further, that “the crenulated condition 
of the articulating upper surface of this plate indicates the 
place of the lower exterior opening into this sac.” 
We are sorry that we cannot agree with Wetherby in some 
of his conclusions. We think the form of the azygous plate, 
on the contrary, indicates that the species had an unusually 
short ventral sac, or actually no sac at all, but was provide 
