NO. IO PHYLOGENETIC STUDY OF RECENT CRINOIDS CLARK 5 



calyx plates, but arm plates. The true calyx plates are (i) the 

 basals, corresponding to the genitals in the urchins, and (2) the 

 infrabasals, corresponding to the echinoid oculars. The radials, 

 which always retain traces of an ultimate origin from two fused 

 plates, are in practically all types the basic .plates of the arms ; but 

 possibly they were originally the second arm plates, for in many of 

 the older types there occur beneath one or more of them, most com- 

 monly under the right posterior, small additional plates which sepa- 

 rate them from the infrabasals. This small plate beneath the right 

 posterior radial is known as the radianal ; in the young comatulid 

 the same plate, which usually appears at a greater or lesser distance 

 from its original position, has almost universally been designated 

 as the anal, though it does not correspond to the anal of the older 

 types. 



As the calyx, through specialization by atrophy, decreases in size, 

 the arms which, being composed dorsally of an extension of the 

 heavily calcified dorso-lateral wall of the calyx and ventrally of an 

 extension of the ventral surface of the disk which draws out along 

 these skeletal supports prolongations from the various ring systems 

 about the mouth, are necessarily situated where these two divisions 

 of the body surface join, cannot accompany the radials in their dis- 

 talward migration. The increasing gap between the radials and the 

 arm bases is therefore filled by a pair of apparently new plates of 

 which the outer, almost invariably axillary, is a double plate, a very 

 close duplication of the radials, but with the two original elements 

 less completely fused, while that between it and the original radial 

 possibly represents the original subradial. The forms with the 

 division series composed of paired ossicles (such as the species of 

 Endoxocrinus for example) thus possess between the radials and the 

 arm bases a series of paired plates, the inner plate of each pair resting 

 upon the radial itself, or upon the outer plate of a preceding pair, 

 and the outer plate of each pair being a reduplication of the original 

 radial. Thus these paired plates of which the division series are 

 formed in most of the later crinoids are not in any way new struc- 

 tures, but an adaptation through a system of reduplication, involving 

 a complicated twinning process, of plates of fundamental signifi- 

 cance common to all crinoids. The formation of the division series 

 of paired plates is exactly comparable to the formation of the column 

 in the pentacrinites, which involves a continuous linear repetition of 

 the complete original column, each unit corresponding to the original 

 column resting upon a cirri ferous nodal as a terminal stem plate, 

 and terminating itself in a cirriferous nodal, which, though in origin 



