XX 



FOSSIL CR1NOIDS 



595 



trifling point of meristic variation comparable to the varying 



number of rows of plates in the interradial areas of the older 



Echinoidea ; and lie is equally sceptical as to the validity 



of Jaekel's division of the group into Cladocpjnoidea and 



Pentacrinoidea, leading to the view that organs like pinnules 



represent totally different structures in different groups. 



Wachsmuth and Springer adopt as bases of classification the 



extent to which the arms and their branches are incorporated in 



the disc, and they 



recognise three main 



divisions: Inadunata, 



in which the arms are 



completely free from 



the calyx; Articulata, 



in which the arms are 



partly incorporated but 



the tegmen remains 



flexible ; and finally 



Camerata, in which 



the arms and their first 



branches are largely 



incorporated in the 



cup ; the tegmen is 



converted into a rigid 



dome and the ambu- 



lacral grooves on it FlG - 274 



become closed, as does 



the mouth, by the meet 



: -^r^g^O^S^, 



Crotcdocrinus pulcher. x 1. B, basal; Br, 

 arm- fan of adhering branches ; col, ossicle of stem ; 

 IB, infra-basal ; R, radial. (After Zittel.) 



ing of overarching folds ; the grooves remaining, of course, open 

 in the distal portions of the arms (Fig. 273). This classification, 

 founded as it is on physiological factors, seems to the present 

 author more satisfactory. Speaking generally, the points in 

 which fossil Crinoids may differ from living genera are: (1) the 

 total absence or irregular nature of the branching in the arms, 

 so that pinnules may be said to be absent ; (2) the closure of 

 the ambulacral grooves and mouth already alluded to, and (3) 

 the adhesion of the arms in the same raj to produce net-like 

 structures (Crotalocrinus, Fig. 274), or a fan-shaped structure 

 (Petalocrinus) ; (4) the frequent presence of two rows of 

 brachials in one arm (biserial structure) ; (5) the develop- 



