Summit Struclure of Pentremites. 495 



to find specimens usually referred to the same genus, dif- 

 fering materially from each other in the construction of 

 their superior parts, when the structure of their separated 

 calyces gives no indication of generic differences, and in 

 some cases approaching each other so nearly as to make 

 it necessary to use the superior parts exclusively for spe- 

 cific distinction. This difficulty is very great in the clas- 

 sification of the Plati/crinidce, and still greater in the Cp- 

 athocrinidcc. Although the structure of the calyx in the 

 latter family is not so simple as that of the former, yet it 

 proves to be incapable of giving expression to the numer- 

 ous generic and specific characters which the family is 

 known to include. 



No instances of a greater number than two primary 

 radials to each ray of Platycrinus have been observed ; yet 

 beyond this there are modifications in different species 

 which seem to be of even more than specific importance. 

 These, however, until lately, have been seldom seen ; and 

 relying on the modifications of the calyx alone, it is to be 

 feared that species have heretofore been founded upon what 

 are really only individual variations, and that in other 

 cases the descriptions were necessarily so meagre that 

 they would apply to specimens which their accompanying 

 arms, discovered after the descriptions were published, 

 prove beyond question to belong to different species, 

 although no one would have been justified in regarding 

 them as distinct, upon an examination of their calyces 

 alone. The same remarks are equally true of the other 

 more prominent genera. 



It is also equally true that genera have been founded 

 upon a misconception of characters, and science has been 

 burdened thereby with synonymes ; but discoveries of more 

 and more perfect forms of these fossils seem to indicate as 

 plainly the propriety of establishing new genera, as the 

 necessity of abandoning some of those which have been 

 proposed. 



