77 



credited to the Kaskaskia Group, whea it was described, but we 

 are uow of the opinion that it came from rocks of the age of the 

 Warsaw or St. Louis Groups, and that, so far as oiir knowledge 

 now extends, the family of the Actinocrinidcv, during the age of 

 the St. Louis Group, ceased to exist. 



We have mentioned eighteen genera belonging to the Aclino- 

 crinida', which are more than belong to any other family of 

 crinoids; and we have enumerated four hundred and fifty-two spe- 

 cies belonging to it, which are more than belong to any other 

 family. We have called attention to the geological distribution of 

 the genera and species and also to some of the distinguishing 

 characteristics. What conclusions, if any, are to be drawn from 

 the facts thus set forth? 



It appears that Saccocrinus, the first genus to appear on the 

 stage, grew to the largest size, developed fifteen species, bearing 

 from ten to twenty ambulacral openings to the vault, and that the 

 arms bifurcate, in Saccocrinus speciosus, twice after becoming 

 free. (See Pal. N. Y., vol. 2, PI. XLVI, fig. 1). The same kind of 

 arms and the same method of bifurcation may be seen in Amphor<i- 

 crhiiis vimincdis from the Keokuk Group. (See North American 

 Geology and Pahtontology, p. 222; and Ohio Pal., vol. 2, PI. II, 

 figs. 12 and 13). And the same kind of arms and the same 

 method of bifurcation may be seen in Aciinocrinus cfibsoni, from the 

 Keokuk Group. (See Bull. No. 3, 111. St. Mus. Nat. Hist., PI. 

 II, fig. 1). And the same may be seen in other genera and spe- 

 cies, in large collections, which have not been illustrated. It fol- 

 lows, therefore, that bifurcating arms composed of a double series 

 of interlocking plates is not a feature of generic importance. This 

 might be proved again by the single and double arms, in the 

 same specimen, in different species of Baiocrinus, illustrated on 

 PI. I, in Bull. No. 7, 111. St. Mus. Nat. Hist. The number of 

 ambulacral openings to the vault is not of generic importance, not 

 only as shown by Saccocrinus, but by Mecfistocriniis, Baiocrinus 

 and other genera that include numerous species. 



The largest forms and very small ones appeared abruptly in 

 Saccocrinus and Ci/licocrinus, in the Niagara Group, as perfect, in 

 all their parts, so far as we know or have reason to believe, as 

 any that existed in later periods; and they became extinct as 

 abruptly as they appeared, in the same geological age. There are 

 no intermediate forma — from these genera to Gennceocrinus and Me- 



