MONOGRAPH OF THE EXISTING CRINOIDS. 



269 



This, however, is susceptible of ready explanation. In the comatulids and 

 pentacrinites the infrabasals have entirely lost their primitive character as impor- 

 tant calyx plates forming an important part of the body wall, and have become 

 entirely negligible constituents of the calcareous structure of the organism. In the 

 comatulids, when they are present at all, after their first appearance they soon fuse 

 with the proximale to form the centrodorsal, and in the pentacrinites they form merely 

 an insignificant circlet of minute plates within the inner ends of the basals. In the 

 ancestors of these groups they were large and important constituents of the calyx, 

 SP important, in fact, that as a result of their apical situation they controlled the 



FIG. 311. 



FIGS. 310-311. 310, THE ARM BASES, CENTRODORSAL ANP CIRRI OF A SPECIMEN or NANOMETRA BOWERSI FROM SOUTHWESTERN 

 JAPAN, ILLUSTRATING THE VARIOUS TTPES OP CIRRI. 311, CIRRI FROM A SPECIMEN OF FLOROUETRA UARLE FROM SOUTHERN 

 JAPAN; (a) A PERIPHERAL AND (6) A SUDAPICAL CIRRUS. 



orientation of the columnals which, originating as rings just beneath the infra- 

 basals, were not able to maintain their primitive circular shape through uniform 

 accretion all around the edges, but were forced to delay their radial growth beneath 

 the convex dorsal surface of the infrabasals, while extending themselves with great 

 rapidity interradially in the slight depressions over the sutures between them. At 

 the same time the encroachment of the infrabasals upon the dorsal opening in the 

 calyx caused the lumen of the growing column to become more or less pentagonal 

 in outline, its angles coinciding with the outer angles of the columnals, so that there 

 was formed a strongly pentagonal column with a more or less pentagonal central 



