1881.] NATURAL SCIENCES OF PHILADELPHIA, 229 



A close comparison of the three sub-divisions of the Palaeocri- 

 noidea shows that the modifications which each undergoes, some- 

 times in the same geological epoch, are moi-e or less repeated in 

 all of them. This is particularlj^ the case as to the construction 

 of the arms. The same development from single to double arm 

 joints occurs in all of them, and simultaneously in the Upper 

 Silurian. A similar analog}' is found in the arrangement of the 

 arms. In each group there are simple and branching arms, arms 

 which are given off directly from the bod}', or laterally from free 

 rays, and even the pinnules are arranged in a like manner. The 

 plates of the bod}^ are composed essentially of the same elements, 

 but in the Platj^crinidse comparatiA'ely few plates are enclosed 

 within the cal3'x, many of them, which in the Actinocrinidse form 

 a conspicuous part of the body, being here found in the lateral 

 appendages. In the Platj'crinidse, the calyx proper is constructed 

 almost exclusively of basals and first radials, all higher orders of 

 radials either forming a part of the brachial appendages, or, when 

 partiall}'^ incorporated with the calyx, being insignificant com- 

 pared with the other parts. In this respect thej' exhibit a marked 

 difference from the Actinocrinidse and Rhodocrinidte, in which the 

 higher radials are prominent elements in the calyx. In the Plat}'- 

 crinidse, the interradial plate is pushed into a line between calyx 

 and dome, and appears like a dome plate, though being in fact a 

 part of the aboral side, and analogous with the first interradial 

 in the Actinocrinidi:e, like this it rests upon the upper edges of 

 the two adjoining first radials, which are generally notched for its 

 reception, a position very different from that of the interradial 

 dome plates. In Dichocrinus the radials are not notched, and 

 the plate in question is actually pushed into the dome, but here 

 also, as in all similar cases, a more profuse development of second 

 and third radials within the calyx, would place this plate in posi- 

 tion with the interradials in the Actinocrinidee. 



We have stated in our family diagnosis that the Sphseroidocrinidae 

 have at least one ijiterradial plate; Pterotocrinus seems to be an 

 exception to this rule, but in that genus the family relations are 

 otherwise so clearly expressed, that it seems to us unnecessary to 

 separate it on account of the absence of that plate. Pteroto- 

 crinus is the last survivor, and probabl}^ the most mature and ex- 

 travagant form of the family In its typical species there are 

 not onl}^ secondary, but also tertiary radials enclosed in the 



