678 HERRICK E. WILSON 



ii. Theory of atrophy and compensating hypertrophy: This 

 theory has been fully considered in the preceding discussion of the 

 formation of the tripartite base in the Batocrinidae and need not 

 be restated. It requires the interpolation of an anal plate in the 

 radial cycle, partial atrophy of the right hah of the posterior basal, 

 a compensating enlargement of the right-posterior basal to bring 

 the right-posterior basal suture into contact with the anal plate, 

 and perhaps the closure of one or more of the primary sutures. 

 However, when this theory is applied to the supposed phylogenetic 

 succession resulting in the Hexacrinidae. a peculiar difficulty is 

 encountered. The Platyeri)ius type of base {ab — c — de — ) in chan- 

 ging to the subequally tripartite base {ab — ex — de) demands upon 

 this theory the reappearance of the right-anterior basal suture. 

 Sutural reappearance as a phylogenetic character is. however, not 

 considered a probability, and the second theory also seems inade- 

 quate to meet the demands of this problem. There is. however. 

 another phase of the problem which must not be overlooked, and 

 that is the possibility that Hexacrinus and Arthroeantha did not 

 originate from one of the Platycrinidae with an ab — c — -de — type 

 of base. This suggestion is not to be considered as a means of 

 confusing the issue and saving the writer's hypothesis: it is inserted 

 to call attention to the fact that there is practically nothing known 

 about the ancestors of the Platycrinidae nor the predecessors of the 

 Silurian genera belonging to that family. The evidence derived 

 from sutural reappearance by delayed anchylosis in the Batocrinidae 

 and Hexacrinidae is such that there seems to be but one possible 

 conclusion to be drawn, winch is that Hexacrinus and Arthrocantha 

 probably originated, not from a three-basal form of the ab — c — de — ■ 

 type, but from a simple five-basal type. The changes required in 

 the suggested line of descent consist of: interpolation of the anal 

 plate by portional migration: closure of the anterior and left- 

 posterior basal sutures: partial atrophy of the posterior basal: 

 and compensating hypertrophy of the right-posterior basal to shift 

 the right-posterior suture to a posterior position (Fig. 0. Xos. 

 1. ;c7. &). 



b"} Evolution of the bipartite base. — Wachsmuth and Springer's 

 theory 1 for the origin of the bipartite, hexagonal base is stated as 



1 Ret". 39, p. 56. 



