BASAL PLATES IN CRINOIDEA CAMERATA 677 



tite bases are obviously so different that it is better to consider 

 their basal developments separately. 



a) Evolution of the tripartite base.— As the evolution of the 

 tripartite base in the Hexacrinidae has resulted in essentially 

 the same type of base as in the Batocrinidae, the same theories 

 of descent will be considered; these are Wachsmuth and Springer's 

 theory of torsion, and the theory of atrophy and compensating 

 hypertrophy. 



i. Torsion theory: Wachsmuth and Springer's theory for the 

 evolution of the tripartite, hexagonal base of the Hexacrinidae is: 

 Upon interpolation of the anal plate in some form having the 

 Platycrinus type of base (ab — c — de — ), spreading of the radial 

 cycle at the posterior side was accompanied by a spreading of the 

 basal cycle at the anterior side, thus causing the compound, dextro- 

 lateral basal to shift so that the right-posterior suture came into 

 contact with the anal plate, and the anterior suture into contact with 

 the right-anterior radial, while at the same time compensating growth 

 of the left-anterior basal filled the vacant space. This metamor- 

 phosis may be shown diagrammatically by changing formula 

 ab — c — de — to ab — ex — de — (Fig. 1, Nos. 3, 9, 10). The method of 

 development is the same as was postulated by Wachsmuth and 

 Springer for the development of the base in the Batocrinidae 

 and the same objections are in force, but as the relationship of the 

 three-basal Hexacrinidae is apparently with the Platycrinidae, 

 these objections will be reconsidered. 



The strongest objection to the torsion theory is that the stimulus 

 for widening the posterior interradius is due directly to the oblique 

 pressure of the growing hind-gut upon the right-posterior radius 

 and posterior interradius, and cannot therefore produce a spreading 

 between the anterior basals. The second objection is that in the 

 two cases of sutural reappearance, by delayed anchylosis, in Hexa- 

 crinus (PI. Ill, Nos. 5, 6) the right-posterior suture does not appear, 

 although in the specimen (PI. Ill, No. 5), showing the reappearance 

 of the left-posterior sutures it would naturally be expected. Wachs- 

 muth and Springer's theory does not then seem entirely adequate 

 in explaining the formation of this base, and we may consider the 

 second theory. 



