BASAL PLATES IN CRINOIDEA CAMERATA 679 



follows : " The bipartite base is probably derived from the tripartite, 

 which preceded it in time, and x, which in the latter constituted a 

 part of c, is united with de, and ab with c [Fig. 1, Nos. 11, 12]." 

 Thus upon the shifting of plate de the tendency for the enlarge- 

 ment of plate c was inhibited, and a compensating growth of plate 

 de filled the space formerly rilled by the enlargement of plate c. 

 This metamorphosis is shown by changing formula ab — cx—de — 

 to abc — xde — . 



To this theory, however, there are the same objections that 

 were encountered in that of the formation of the tripartite base 

 in both the Batocrinidae and the Hexacrinidae. Torsion such as 

 is assumed for the shifting of the compound plate de is apparently 

 impossible, as the stimulus for enlarging the posterior interradius 

 is due to the pressure from the growing hind-gut and cannot affect 

 the anterior basal sutures. Furthermore, the examples of suture 

 reappearance in the bipartite base show the potential presence and 

 position of all but the right-posterior suture (PL III, Nos. 8, 9, 10). 

 Wachsmuth and Springer's theory again does not seem adequate 

 to explain the changes which have taken place and we may consider 

 the theory of atrophy and compensating hypertrophy. 



When the theory of atrophy and compensating hypertrophy, 

 which has been thoroughly discussed, is applied to the phylogenetic 

 succession as formulated by Wachsmuth and Springer, it also meets 

 with the same trouble that was encountered in considering the 

 evolution of the tripartite base. In this case the anterior basal 

 suture must reappear as a phylogenetic character. Phylogenetic 

 reappearance of a suture lost through anchylosis is, however, con- 

 sidered impossible, and the phylogenetic succession as postulated 

 by Wachsmuth and Springer must be examined. 



There are, it is true, many similar characteristics in the Hexa- 

 crinidae having tripartite and bipartite bases, but when the teg- 

 minal structures of the Hexacrinus and Dichocrinus are compared 

 there is a marked difference. Hexacrinus has a ridged tegmen 

 composed of medium-sized plates, and the ambulacrals are usually 

 partially incorporated. Dichocrinus, however, has in its earliest 

 expression in the Kinderhook a flexible tegmen composed of 

 minute inter ambulacrals and unincorporated ambulacrals. Dicho- 

 crinus therefore apparently could not have originated from the 



