190 C. M. Keyes — Carboniferous Echinodermata 



of the Lower Carboniferous. At the close of the Keokuk 

 epoch one-half of the Carboniferous genera had become 

 extinct. The great group Camerata had passed away, with the 

 exception of the Hexacrinidse and a few depauperate forms of 

 several other genera whose existence was quickly brought to a 

 close. A large proportion of the genera in the extensive sec- 

 tion Inadunata had disappeared ; of those groups which sur- 

 vived to the close of the period, a diminutive species Allage- 

 crinus (a single specimen only being at present known) was 

 the sole representative of the branch Larviformia ; while of 

 the great group Fistulata only the typical genus (including 

 four subgenera) of the Poteriocrinidse extended through the 

 entire Lower Carboniferous. And the widely distributed Cat-, 

 ceocrinus which began back in the Lower Silurian became 

 extinct just before the beginning of the St. Louis. 



"Wachsmuth and Springer* have shown that in the expansion 

 and geological development of the various groups of crinoids 

 the modification of specific characters was very gradual and 

 corresponded in a striking manner with the changes by growth 

 in the individual. Another suggestive fact is that usually the 

 more generalized types of the various groups are the more 

 persistent, often having a considerable range both in time and 

 space. The expansion of the several families is also frequently 

 indicated by the relatively rapid development, in some supra- 

 generic groups, of certain structural features which soon be- 

 come curiously differentiated Perhaps nowhere in any 

 zoological group is its culmination better or more clearly 

 defined, in accordance with the suggestions already made, than 

 in the Crinoidea. The remarkable multiplicity of specific and 

 generic types, appearing in rapid succession during the middle 

 Lower Carboniferous ; the extreme and phenomenal specializa- 

 tion of particular anatomical structures ; the great increase in 

 size ; the ponderous character of the test ; and the marked 

 structural changes in many minor particulars are of peculiar 

 biological significance. Toward the close of the Keokuk 

 nearly all the specialized forms became extinct ; and with a 

 very few exceptions only the more generalized types continued 

 through the Lower Carboniferous — only such forms as were 

 ordinally related to living crinoids. 



If the crinoids formed a prominent faunal feature of the 

 earlier part of the Lower Carboniferous, the blastoids were 

 equally conspicuous during the latter portion of the period. 

 In the Burlington and Keokuk the Blastoidea, although repre- 

 sented by more genera than in the St. Louis and Chester, were 

 for the most part rare individually ; and their presence was 

 rendered still less noticeable by the great preponderance of 



* Proc. Acad. Nat. Sci. Philad., 1878. 



