CRINOIDS. 147 



generally terminatiDg in a rather abrupt extinction of the en- 

 tire group. 



The culmination of crinoidal life as a whole was in the 

 middle of the Lower Carboniferous. In the great interior 

 province, at the close of the Keokuk epoch, one-half of the 

 Carboniferous genera had become extinct. The great group 

 of the Camerata had passed away with the exception of the 

 Hexacrinidcie, and a few depauperate forms of several other 

 genera whose existence was speedily brought to a close. A. 

 large proportion of the genera in the extensive section Inadu- 

 nata had disappeared ; of those groups which survived to 

 the close of the period, a diminutive species of Allagecrinus 

 (a single specimen only being known at present) was the sole 

 representative of the branch Larviformia; while of the great 

 group Fistulata only the typical genus (including four subge- 

 nera ) of ttie Poteriocrinidse extended through the entire Lower 

 Carboniferous. The widely distributed Calceocrinus, which 

 began back in the Lower Silurian, became extinct just before 

 the beginning of the Saint Louis. 



The abrupt extinction of a large proportion of the crinoi- 

 dal forms toward the close of the Keokuk is certainly sugges- 

 tive of a series of decided and widespread changes in the 

 geographic and bathj metric extent of the great interior sea. 

 White has already shown that at least in some portions of the 

 Mississippi province there were very considerable alterations 

 in the coastal contour of this broad shallow gulf, during the 

 latter part of the Lower Carboniferous ; and it is known that 

 there were even greater changes in the coast line in other 

 parts of this region during the same period. 



Numerous attempts have been made at various times to 

 demonstrate that in the expansion and geological develop- 

 ment of the different groups of fossil organisms, the modifi- 

 cation of the specific characters was very gradual, and 

 corresponded in a striking manner with the changes of 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 very considerable range both in time 



