676 HERRICK E. WILSON 



of the right-posterior basal. The formula of the tripartite, hexag- 

 onal base, upon the theory of atrophy and compensating hyper- 

 trophy, is ab — cd — ex — . 



The explanation of the reversion implied in the abnormal 

 specimens of Teleiocrinus umbrosus and Steganocrinus pentagonus 

 may upon this theory be readily explained. Upon loss of the anal 

 plate, stimulus for shifting the posterior basal suture was also lost, 

 but the tendency for anchylosis of the anterior basals established 

 in the early Batocrinoidea was not changed. Anchylosis then took 

 place in as nearly normal a manner as possible, one of the postero- 

 lateral sutures only in each specimen failing to close. 



It will be noted that in the preceding discussion no mention 

 has been made of the proportionate amount of growth in the 

 radials, the anal, and the enlarged basals. This is because some 

 misconception has arisen concerning their development, owing to 

 the assumption that the basal outline is a regular hexagon. From 

 careful measurements of the proximal diameter of the radials and 

 anal in over five hundred specimens of Batocrinidae and Actino- 

 crinidae it was found that in general the largest plates in the 

 radial cycle are the antero-lateral radials, while the postero-laterals 

 or the anal are the smallest in the radial cycle. Since the posterior 

 radials are reduced by the interpolation of the anal plate (see p. 549) 

 and the margins of the posterior basal do not extend beyond the 

 center of these radials, it is evident that the posterior basal did not 

 enlarge to a width equal to that of two of the other basals, as Wachs- 

 muth and Springer have assumed, nor is the base a regular hexagon. 

 The reduction of the posterior basal and the compensating enlarge- 

 ment of the right-posterior basal do not then require a formidable 

 amount of plate readjustment, especially when we remember what 

 readjustments have taken place in Pisocrinus (see p. 535). 



3. EVOLUTION OF THE BASE IN THE HEXACRINIDAE 



The evolution of the base in the Hexacrinidae is not as simple a 

 problem as the evolution of the base in the Batocrinidae, for the 

 phylogenetic succession is not as clearly defined. The general 

 affinities of the Hexacrinidae are with the Platycrinidae, but the 

 paths of evolution resulting in the subequally tripartite and bipar- 



