152 PROCEEDINGS OF THE ACADEMY OP [1890. 



General Relations. There is often considerable embarrassment in 

 attempting to separate certain paleozoic Capnli, on the one hand from 

 some forms of Platysfoma, especially from those species in which 

 there is a greater or less tendency for the shells to nncoil ; and on 

 the other hand from various genera of Patelloid shells. As might 

 be expected in a group of gasteropods presenting so few constant 

 characters, which can be satisfactorily relied upon as classificatory 

 criteria, it is often impossible to clearly distinguish between certain of 

 these species. Many structural features long regarded as of much 

 importance in identification have recently ^ been shown to possess 

 very little, if any, specific value, owing to their great variability. It 

 has therefore become necessary to consider as of the utmost signifi- 

 cance, the basing of species upon general resemblances rather than 

 upon unimportant varient characters arising from the diverse 

 conditions of environment imposed by a more or less extensive geo- 

 graphic and geologic distribution. Therefore in choosing for classi- 

 ficatory purposes the characters of any group it is evident that only 

 those features exhibiting the least tendency to modification are 

 available. Even the most constant structures appear to lose much 

 of their stability at some period during the existence of the group — 

 whether specific, generic or family ; while other characters more or 

 less variant in the earlier stages of development, later become less 

 liable to change. At some time these features blend and thus appear 

 the transitional forms. It may be assumed, then, that in many 

 groups of the same genetic origin some varieties will present features 

 that have remained for a long time practically unmodified ; w^hile 

 others exhibit the same characters in a highly specialized, but ever 

 changing condition. And it is of great interest to note that the 

 latter — those having greatly exaggerated features — are the forms 

 wiiose existence is of comparatively short duration ; and that with 

 these intensified structures the development is rather rapid, while 

 their culmination results in a great diminution of the gi'oup's 

 vitality, or more commonly its extinction. 



Number of Species. Among the first to notice the existence of 

 Carbonic Capuli in the continental interior were Yandell and Shu- 

 mard, who called attention to the association of a species with an 

 Acrocrmus (afterwards described by the former author as A. shu- 

 viardi). These writers attempted to prove that the crinoids were 

 carnivorous in their habits, and that they subsisted on moUusks. 



1 Keyes : Proc. Am. Philosophical Soc, vol. XXV, p. 231. 



