ECHINODERMATA—CRINOIDEA. 491 



various names according to the plates or series of plates which 

 they separate ; those between the R and the succeeding rays are 

 called by the general term interradials (IR, Fig. 1804) whether 

 belonging to the dorsal cup or to the ventral disk ; those of the 

 dorsal cup between the brachials may be distinguished as intcr- 

 brachiah, those of the ventral disk lying between the ambulacra as 

 interamhulacrals, while those between the different orders of bra- 

 chials as intercostals, interdistichals, etc. (interprimibrachs, interse- 

 cundibrachs, etc.). In most Palaeozoic crinoids anal plates form 

 part of the dorsal cup on the posterior side in line with the anus. 

 The first anal plate (IRA) when present rests on the upper face of 

 the posterior brachial B^, thus adding a sixth plate to the row of 

 five R. Even though the IRA may be lacking, auxiliary anal plates 

 may be present, interposed between the interbrachials and follow- 

 ing the median line of the posterior area. The radianal (RA) or 

 second anal plate when present rests in the reentrant angle of two 

 adjoining B to the right of the first anal plate. 



The ventral disk or tegmen consists of the orals, ambulacral and 

 interambulacral plates. The orals are five large interradial plates 

 surrounding the mouth or covering it (Fig. 1810). They may be 

 of uniform size and form, or the posterior plate may be larger and 

 nearly central. The orals may be large, small or indistinguishable. 



Fig. 1805. Cadocrinus proboscidialis. a, calyx with broken vault, X ^^ » ^^'i 

 ambulacral canals ; c, digestive sac ; o, arm openings \ s, " convoluted organ," X ' /^ 5 

 <^, ambulacral canal, magnified. (After Meek and Worthen, 111. Geol., V.) 



The plates of the tegmen are, in general, much less regular than 

 those of the dorsal cup and often are not differentiated into special 

 series. The ambulacral plates consist of the ambulacral or side 

 pieces and the covering plates ; the latter form a roof over the 



