BASAL PLATES IN CRINOIDEA CAMERATA 669 



duplication of the posterior basal; secondly, the locking in of the 

 sixth basal in the basal cycle; and, thirdly, closure of the posteriorly 

 directed basal suture by anchylosis. On the ground of possibilities 

 there can be no objection to this hypothesis, for we know that this 

 certainly occurred in Sagenocrinus, nor can there be serious objec- 

 tion to the locking in of this plate in the basal cycle, for the radianal 

 of Pisocrinus is locked in in the radial cycle. The most serious 

 objection to this theory is that in the series of sutural reappearances 

 in species with a hexagonal base a normal right-posterior basal 

 suture has not been found, although the writer has made careful 

 search for such examples. 



iii. The theory of asymmetrical enlargement requires either 

 enlargement of the left side of the posterior basal, or of the right 

 side, which is Wachsmuth and Springer's theory for enlargement. 

 In favor of the first development is the fact that in the Flexibilia 

 Springer has found a general enlargement of the left side of the 

 posterior basal. 1 In favor of the second part of this theory is 

 the development to the right of the radianal in Pisocrinus, appar- 

 ently in the face of the influence of the growing intestine which in 

 the Flexibilia tends to carry this plate to the left of the right- 

 posterior radial. The abundance of evidence Springer has brought 

 in his work on Flexibilia far outweighs the slight evidence shown 

 in Pisocrinus and inclines one to believe that factor x might have 

 been added to left of basal a; but when we consider the evidence 

 of increasing potency for distortion on the part of the intestine, 

 there is a strong possibility that the intestine may have shoved the 

 posterior basal to the left, as Wachsmuth and Springer believe, 

 and may have at the same time inhibited the tendency of expansion 

 on the part of the right-posterior basal, permitting the posterior 

 basal to enlarge on the right side. We are here involved in a ques- 

 tion of directive controls which cannot be readily answered, and, 

 since the posterior basal in Tanaocrinus is symmetrically trunkated 

 and developed, the writer is forced to the conclusion that asym- 

 metrical development had not at that time appeared in the posterior 

 basal. 



The formula for a base developed upon the first theory is 

 xax — b — c — d — e — , or in shortened form a — b — c — d — e — (Fig. 9, 



1 Ref. 31, p. 496. 



