1870. j NATURAL SCIENCES OF PHILADELPHIA. 255 



In Onychocrinus, the genus which evidently possessed the 

 greatest expansive power, the radial areas being frequently found 

 spread out horizontally, there are toward the inner or ventral side 

 of the rays rather large plates, to which smaller ones are joined, 

 which connect with the interradial series. They decrease in size 

 and thickness inwardly, and connect with the dome plates. In a 

 specimen of Onychocrinus exsculptus, Lyon, and another of 0. 

 diversus, Hall, we found the median portion of the vault preserved, 

 the plates being irregularly arranged, rather large and thin. This 

 important observation goes far toward proving that the Ichthyo- 

 crinidse had no external oral aperture, for if any of the family 

 were likely to have it, it would be Onychocrinus. 



The Ichthyocrinidae are nearest related to the C yathocrinidse, 

 from which they differ in having several orders of radials included 

 within the body ; in the articulate structure of the radial portions; 

 in the presence of interradial plates within the regions of the 

 calyx, and in the pliant vault. 



The separation of the genera in this family has alwaj^s been 

 attended with difficulty, and it cannot be denied that several of 

 them shade into one another in a most perplexing manner. They 

 are very closely related, and yet there is a habitus, peculiar to 

 typical forms of each genus, which is not easity described, but 

 which is readily perceived when large collections are brought 

 together, and which Paleontologists discerned at an early day. It 

 is the gradations the transition forms which make trouble, and 

 have given rise to continual modifications of the generic formulae 

 in hope of reconciling them with new discoveries. While we can- 

 not expect that the divisions we have made are wholly free from 

 errors, yet we find when we arrange the different species according 

 to the generic characters herein given, that the groupings are 

 more nearly conformable to the recognized habitus of the fossils 

 than any we have been able to make heretofore, and we are en- 

 couraged to hope that we may have approximated more closely to 

 the natural divisions. 



The Ichthyocrinidse range from the Lower Silurian to the close 

 of the Subcarboniferous. 



We recognize the following genera : 



