FOSSIL CRINOIDS 



595 



tiitiing point of iiieristic variation comparable to the varying 

 niiml)er of rows of ])lates in the interradial areas of the older 

 Eeliinoidea ; and lie is equally sceptical as to the validity 

 of Jaekel's division of the group into Cladocuinoidea and 

 Pentackinoidea, leading to the view that organs like pinnules 

 represent totally different structures in different groups. 

 Wachsnmth 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 ^^^^'- 2"^:. 

 become closed, as does 

 the mouth, by the meet- 

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

 in the distal portions of tlie 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 ray to produce net-like 

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

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

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



-Crotalocrinus piiJchcr. x 1. B, basal; £>r, 

 an of adhering branches ; co/, ossicle of stem ; 

 //!, infra-basal ; E, radial. (After Zittel.) 



