WOOD, CKINOID ARMi^ IN STUDIED OF PHYLOQENY 15 



It seems reasonable to assume that C. ccelatus var. spinotentaculus is a 

 direct descendant from C. muUihracliiaius, differing but little from the 

 latter, its immediate ancestor. I have not seen the arms of Cactociinus 

 ccelatus; hence comparisons are made with its variety, of which well-pre- 

 served material is available. 



Cactociinus limabrachiatus resembles C. multibrachiatus in the form 

 of the calyx and in the changes in form through which the arms pass. 

 The ornament on the surface of the calyx and the arms of the former 

 species is similar in kind to that of the latter, but is more extreme, the 

 costae being stronger and more numerous and the corrugations of the 

 arms coarser. In this respect, it is more highly differentiated than ('. 

 inultibrachiatus, but in the number of arms and in the late attainment 

 of the biserial condition it is more primitive. My interpretation of these 

 facts would be that both are descended from a common ancestor but rep- 

 resent divergent lines of evolution, Cactocrinus Umahracliiatus having 

 developed in the direction of a high degree of surface ornament, while 

 C. multibrachiatus has advanced in number of arms and earlier develop- 

 ment of biserial plates. 



Relation of Cactocrinus to Teleiocrinus 



'J^he derivation of the genus Tehiocnnus from Cactocrinus has already 

 been suggested by Wachsmuth and Springer,^ and it is interesting to note 

 that the development of the arms confirms the evidence derived from the 

 calyx and the mode of branching of the arms. 



In Teleiocrinus unibrosus (Hall), the type of the genus, the arms are 

 more numerous and more slender than those of Cactocrinus ccelatus var. 

 spinotentaculus. Their slenderness is perhaps due to their greater num- 

 ber, both on account of economy of material and the crowding due to fre- 

 quent branching near the base. The method of branching in Teleiocrinus 

 follows the Cactocrinus plan, each axillary being succeeded by another 

 without intervening plates of the same order until there are fifteen or 

 sixteen arms to the ray. In form, the arms are cylindrical at the base, or 

 in accelerated individuals somewhat flattened even at this point. Higher 

 up on the arms, they become strongly flattened dorso-ventrally and de- 

 velop a narrow transverse ridge near their upper margins. The corruga- 

 tions on the surface of the arms are much finer than those of Cactocrinus 

 ccelatus var. spinotentaculus, as might be expected from the more delicate 

 structure of the whole arm. The arm of Teleiocrinus umbrosus is figured 

 on Plate V, figs. 5, 5a. A comparison of these figures with those of 



^Wach.smuth and Springkr : North American ("rinoidea ('amerata, p. fi27. Cam- 

 bridge, 1807. 



