284 



CRINOIDEA. 



the centre of its base is formed by a single plate, which may 

 be regarded as the uppermost segment of the stem, or as an 

 azygous basal, or as formed by the anchylosis of several 

 basals. Bound this are two cycles of parabasals (or one 

 cycle of basals and one of parabasals), surmounted by a 

 single cycle of radials. There are five arms, and the upper 



Fig. 170. A, Side view of the calyx and bases of the arms of Marsiipiles ornatus, from the 

 Chalk. B, Diagram (after Hall) of the dissected calyx of Agassizocrinus: &, Basals ; p, Para- 

 basals ; r, Radials ; a, Anal plates. 



surface of the calyx appears to have been covered by a 

 leathery membrane, containing calcareous ossicles. Mar- 

 supites is wholly confined to the Cretaceous ; and the allied 

 genus Uintacrinus, with ten arms, is from rocks of the same 

 age in North America. In the curious Astylocrinus and 

 Agassizocrinus, of the Carboniferous, we have two closely 

 allied, or identical, Palaeozoic types of sessile Crinoids. In 

 these forms (fig. 170, B) the base of the calyx is formed of 

 five anchylosed basals, succeeded by five parabasals, which in 

 turn are surmounted by two cycles of radials. Agassizocrinus 

 appears also to possess a small number of anal plates. 



Finally, in the family of the Comatulidce, represented by 

 our living Feather-stars, we have Crinoids, which are pedun- 

 culate in the early stages of their existence, but which in the 

 adult state lead an entirely free life. In their free condition, 

 the " peduncle " of the stalked Crinoids is represented only 



