BASAL PLATES IN CRINOIDEA CAMERATA 68 1 



in Coccocrinus, etc., and to the Stephanocrinus type of base 

 (ea — be — d — ■) in Hapalocrinus. Anchylosis of the basals in these 

 forms was accompanied by reduction of the compound plate either 

 by superficial or by deep-seated atrophy, and compensating hyper- 

 trophy of the simple basal. From Coccocrinus two lines of descent 

 seem to have originated: the one culminating in Platycrinus, etc., 

 with the ab — c — de — ■ (Fig. 9, No. 4) type of base and an elliptical 

 stem; the other culminating in Dichocrinus with its hexagonal, 

 bipartite base (abc — dex — ■) (Fig. 9, No. 46) and a circular stem. 

 Interpolation of the anal plate in Dichocrinus is marked by reduc- 

 tion of the posterior radials, and the base is not a regular hexagon. 

 The origin of the hexagonal genera with the tripartite base ab — cd — 

 ex — (Fig. 9, No. 7&) is doubtful, but they were probably derived 

 from an early branch of the primitive Platycrinidae, in which the 

 base was a simple pentagon of the a — b — c — d — e — type and the 

 stem circular. 



SUMMARY OF CONCLUSIONS 



In the preceding discussion the writer has attempted to por- 

 tray the various stages of development through which the basal 

 plates of the monocyclic Camerata have passed, and to explain 

 the processes by which these results were obtained. Many of the 

 theories herein set forth will undoubtedly be modified by the dis- 

 covery of new evidence, and this study is offered merely as a 

 working hypothesis upon which, perhaps, a stronger classification 

 of the Crinoidea may be erected. As a result of this study the 

 following conclusions have been reached: 



1. The ancestor of the monocyclic Camerata was a simple, 

 generalized crinoid with pentamerous symmetry. 



2. The base of the pentagonal Camerata is not the result of 

 reversion from an intermediate hexagonal stage, but is in a primi- 

 tive condition as far as influence of the anal plate is concerned. 



3. The anal plate is of secondary origin, and originated by 

 primary interpolation between the latero-distal margins of the 

 posterior radials. 



4. The hexagonal base of the monocyclic Camerata resulted 

 from the separation of the posterior radials and trunkation of the 



