78 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The plates of the first circlet are usually reduced in number and may be 

 quite absent; those of the second circlet are often reduced in number, and may 

 be highly metamorphosed. 



In the echinoids the coronal plates are always large and conspicuous, 

 forming a ring of 10 plates about the periproctal area, 5 small (the oculars, 

 corresponding to the infrabasals) and 5 large (the genitals, corresponding 

 to the basals). 



Costal axillary (IBr 2 ). The first axillary following the radial; the primibrachial 

 axillary; by the older authors this was called the radial axillary (see figs. 1, p. 

 60, 3, p. 62, 29, 30, p. 71, and pp. 109, 110). 



Costal pinnules (P c ). The pinnules borne by the costals or primibrachs; among 

 the recent comatulids these occur only in the genus Eudiocrinus, where the 

 second costal or primibrach (IBr 2 ) is not an axillary as usual, but bears a 

 pinnule instead of an additional arm (see figs. 83, p. 136, 84, p. 137 andpp. 114, 115). 



Costals (IBr). The postradial ossicles as far as the first axillary; the ossicles of the 

 first division series; the primibrachs; in all the recent crinoids except Meta- 

 crinus these are two in number, and, except in Eudiocrinus, terminate in an 

 axillary; they are not found in the PentametrocrinidaB (see figs. 1, p. 60, 3, 

 p. 62, and 29, 30, p. 71, and pp. 109, 110). 



Though similar in appearance, the first division series is not homologous in 

 all types. 



Covering plates. Thin rounded calcareous plates developed along the borders of 

 the ambulacral grooves and capable of being closed down over them; in pre- 

 served specimens they are easily visible with a hand lens of low power as a 

 series of oval or approximately circular alternating 'imbricate plates concealing 

 the ambulacra; covering plates are almost invariably associated with side 

 plates (see figs. 7, p. 63, and 55, p. 81). 



Among the comatulids covering plates are usually rudimentary or absent 

 except in the families Thalassometridas, Charitometridse and Calometridse ; 

 they are also large and well developed in certain of the Heliometrinse, and in a 

 few of the Capillasterinse, in the latter occurring without side plates. 



Crenellx. Narrow rounded ridges, arranged more or less radially, most fre- 

 quently observed upon the joint faces of columnals and, in the comatulids, 

 upon the apposed faces of two brachials united by syzygy (see figs. 34, p. 71, 

 and 525, pi. 1, and pp. 208-210). 



Crenulate sutures. Sutures which are evident externally as a wavy line (see 

 figs. 127, p. 197, and 128, p. 199). 



Crown. The calyx and arms; a crinoid deprived of its column; this term is not 

 applicable to the comatulids. 



Cryptosynartliry.A. synarthry which has become modified by a general flattening 

 of the joint faces, together with a restriction in the proportionate area occupied 

 by the ligament fossae, so that the latter appears very small; typically a crypto- 

 synarthry shows a very nearly plane articular surface upon which the position 

 of the central dorsoventral ridge may with difficulty be traced (see fig. 36, p. 75 

 and p. 113). 



