546 HERRICK E. WILSON 



sidered as a gentle pressure, nor the displacement of these plates 

 as a gentle process, due only to their much closer association with 

 the intestine than with the surrounding plates. 1 The outward 

 push of the intestine is in proportion to the strength of the calyx 

 walls a powerful force, capable of inhibiting plate growth and of 

 greatly distorting the relations of plate contact and position. 



In the recent works of Springer and Clark attention has been 

 called repeatedly to the remarkable parallel between the ontogenetic 

 migrations of the radianal in modern crinoids and its phylogenetic 

 migration in fossil forms. Since the migration of the radianal in 

 recent forms is caused directly from its intimate association with 

 the hind-gut in its upward growth, there can be no doubt that such 

 an association existed in the ancient crinoids, and that the tendency 

 for shifting the radianal gradually increased and the association 

 became so firmly established that the radianal is now completely 

 withdrawn from the cup in individual development. 



The radianal in the early Flexibilia is incorporated in the basal 

 cycle below the right-posterior radial, and probably appeared in 

 that position in the ancestors of the Flexibilia. 2 The outward push 

 of the developing intestine was then directed obliquely to the 

 right against the radianal and in the succeeding stages shoved and 

 pulled this plate upward and to the right into the posterior inter- 

 radius and out of the cup. Furthermore, in Sagenocrinus 3 it 

 permitted such an enlargement of the radianal that the right mar- 

 gin of the posterior basal was shoved to the center of the posterior 

 interray. This change is of especial interest in the study of basal 

 plate evolution, as it shows one method of obtaining a posteriorly 

 directed basal suture, such as is exhibited in all genera of Camerata 

 having a hexagonal, tripartite base. 



This apparent digression from the subject of basal plate evolu- 

 tion in the Camerata — a group in which, as far as we now know, a 

 radianal plate never appeared — is for the purpose of bringing 

 clearly to mind the powerful effect of the growing intestine and the 

 presence of zones of potential weakness in the calyx. These zones 

 of weakness lie along the posterior interradius in the radial cycle 



1 Ref. ii, p. 732. 



2 Ref. 31, p. 439, PL V, Fig. 9. » Ibid., PI. VII, Fig. 18. 



