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HERRICK E. WILSON 



of the posterolateral basals is free, as in ea — b — cd — , or ab — cd — e — 

 The probabilities are, however, that the three-basal forms originated 

 directly from the live-basal forms, without an intermediate four- 

 basal stage. This succession is shown diagrammatically by chan- 

 ging formula a — b — c — d — e — to ab — c — de — for the Platycrinus 

 type of base and to ea — be — d — for the Hapalocrinns type of base 

 (Fig. 9, Nos. i, 3, and 4). Two-basal forms are not known in this 



4a 



Fig. 9. — Diagrams illustrating the evolution of the base in the Camerata on the 

 theory of atrophy and compensating hypertrophy: 1-5, combinations of the primi- 

 tive five-basals; 1, 6, 7-76, succession in the Batocrinidae; 1, yb, succession in forma- 

 tion of the tripartite base in the Hexacrinidae; 1, 4-46, succession in the formation of 

 the bipartite base in the Hexacrinidae. 



succession, so the next stage is that of complete anchylosis (Fig. 9, 

 No. 5). This change consists in the complete union of the basal 

 plates, and may occur in either the live-, four-, or three-basal forms. 

 The formula for complete anchylosis is abede. 



2. THEORIES FOR THE EVOLUTION OF THE BASE IX HEXAGONAL CAMERATA 



a) Enlargement of the posterior basal. — Having in mind the time 

 and place of development of the anal plate, let us examine the 



