152 CRINOIDS. 



cially on the posterior side, where a vertical row of anal plates 

 is very noticeable. Ventrally the calyx is very protuberant, 

 and sometimes inflated not unlike that in Amphoracrinus. 



Dorycrinus is the direct lineal successor of Gennseocrinus, 

 from which it should, perhaps, not be separated generically. 

 The anal structure links it closely with Agaricocrinus. It 

 differs, however, in having the general arrangement of the 

 calyx more like Batocrinus, and in a less massive arm structure. 

 The long spines, so conspicuous on the ventral plates of some 

 species, seem to be merely greatly exaggerated developments, 

 homologous with the large nodosities on similar plates in 

 Agaricocrinus. 



Extreme forms of Eretmocrinus differ from those of Bato- 

 crinus principally in the long lanceolate arms and inflated 

 ventral parts, besides usually a more or less well-defined lateral 

 extension of the basals. It is manifestly an offshoot of Bato- 

 crinus, for the gradations are very complete, and there is often 

 considerable difiQculty in separating the forms of the two 

 groups. The genus was rather short-lived, appearing in the 

 Burlington and becoming extinct before the close of the 

 Keokuk. 



In Batocrinus the long anal tube, like that of the typical 

 form of the family, is very prominent. The arms are short. 

 The plates in the second tier of the anal interradius are three 

 in number ; orals, large and well defined. Batocrinus is one 

 of the most characteristic and widely-spread types of the family 

 occurring in the Lower Carboniferous. Its relations to the other 

 genera have already been considered elsewhere and need not 

 be repeated here. 



Actinocrinus, and the genera following, have only two 

 pieces in the second anal tier. In the leading genus two rather 

 well-marked sections are recognizable : one with the arms in 

 clusters, imparting a strongly quinquelobate symmetry ; the 

 other with the arms equidistant around the margin of the calyx. 

 The small number of brachials below the free arms is also very 

 noticeable when compared with the four groups yet to be con- 

 sidered. 



