674 HERRICK E. WILSON 



implied is very close to being a solution for the problem of the 

 appearance of the posteriorly directed basal suture, and we need 

 no longer confuse the issue with origin from Platycrinus or other 

 three-basal forms of the ab — c — de — ■ type. 



It seems then that Wachsmuth and Springer's theory for the 

 development of the hexagonal, tripartite base does not meet 

 the requirements of present needs, and we may turn to a discussion 

 of the remaining theories. 



ii. Bisection theory: Briefly stated, the bisection theory 

 assumes that the posteriorly directed basal suture resulted from 

 the bisection of the posterior basal in a quadripartite base, and 

 anchylosis of the portions of the posterior basal to the adjacent 

 basals. This development may be expressed in formula as follows: 

 a — b — cd — e — through a — b — cd — e — a — to ab — c — d — ea — . The 

 first objection to this theory is that there are no known instances of 

 the bisection of a growing plate in modern Echinodermata. The 

 second objection is that in the examples of recurrence of sutures by 

 delayed anchylosis there are no instances of the reappearance of a 

 normal right-posterior basal suture when the posterior suture is 

 present (see Pis. II, III), although the writer has, as previously 

 stated, made careful search for such examples. This theory and 

 any theory based upon the assumption of plate splitting as a phylo- 

 genetic characteristic may apparently be abandoned as a factor in 

 the development of Crinoidea. 



iii. Theory of atrophy and compensating hypertrophy: The 

 theory of atrophy and compensating hypertrophy is that the 

 posteriorly directed basal suture and the subequally tripartite base 

 of the hexagonal, monocyclic typical Camerata arose from the 

 atrophy of the right half of the enlarged posterior basal, a compen- 

 sating enlargement of the right-posterior basal, and the anchylosis 

 of the posterior and left-posterior basals. 



This theory is based upon (i) the general presence of atrophy 

 in the right side of the posterior basal in Flexibilia; (2) the non- 

 appearance of the right-posterior basal suture in the specimens 

 showing reappearance of lost sutures through delayed anchylosis, 

 and (3) evidence showing the derivation of Dichocrinus from 

 Platycrinus stock. 



