212 MANUAL OF ZOOLOGY. 



origin to the plates of the arms. The radial plates are ar- 

 ranged in a series of vertical columns, which radiate from the 

 summit of the basals to the bases of the arms. Between the 



Fig. 104. Diagram of the dissected calyx of Rhodocrinus, viewed from below (after 

 Schultze). b Basals ; / Parabasals ; r First radials ; t Inter - radials ; a Anal 

 plates. 



different columns of radial plates, however, there may be inter- 

 calated certain other smaller plates, which, from their position, 

 are termed " inter-radials " (fig. 104, /); while one of the inter- 

 radial spaces, corresponding with the anus, is usually much 

 wider than the others, and is furnished with an additional series 

 of calcareous pieces, which are termed "anal plates "(fig. 104, a). 

 Of the living stalked Crinoids, the best known is the Penta- 

 crinus caput - Medusa of the Caribbean Sea. Another West 

 Indian form is the curious sessile Holopus. More recently a 

 stalked Crinoid has been discovered in the Atlantic and North 

 Sea, and has been described under the name of Rhizocrinus 

 Lofotensis (fig. joi). The chief interest of this form is the fact 

 that it belongs to a group of the Crinoidea hitherto believed 

 to be exclusively confined to the Mesozoic rocks viz., the 

 Apiocrinida or " Pear-encrinites." In fact, Rhizocrinus is very 

 closely allied to the Cretaceous genus Bourgueticrinus, and it 

 may even be doubted if it is generically separable from it. The 



