88 



ECHINODERMATA. CRINOIDEA. 



which case the plates are placed vertically above one 

 another. In some genera there are between the rows of 

 radials, other smaller plates, the inter-radials (fig. 21, ir) ; 

 and in the inter-radius which leads up to the anus there 

 is an extra series of plates, the anal inter-radials (fig. 21, a). 

 From the highest row of radials the arms arise ; these are 

 generally movable, they have a groove on the upper 

 surface, and are formed either of a single row of plates, 



FIG. 22. Apiocrinus parkinsoni, from the Bradford Clay, s, topmost 

 plate of the stem ; b, basal plates ; r 1, r 2, r 3, first, second and 

 third cycles of radial plates; br, brachial plates. (Three-quarters 

 natural size.) 



the brachials (br), or of a double row, in which case the 

 plates alternate. The pinnules also consist of calcareous 

 plates, with an arrangement similar to those of the arms. 

 It has already been stated that the upper surface of 

 the disc is sometimes furnished with small plates, but 

 these are seldom preserved fossil ; they do not how- 

 ever cover over the food grooves or mouth, which are 

 still visible externally. But in most of the Palaeozoic 

 crinoids there is also a more or less complete roof of 



