THE CARBONIFEROUS PERIOD. 



175 



disappeared, and it is never again represented save sporadi- 

 cally and by isolated forms. 



Amongst the Echinoderms, by far the most important forms 

 are the Sea-lilies and the Sea-urchins the former from their 

 great abundance, and the latter from their singular structure ; 

 but the little group of the " Pentremites " also requires to be 

 noticed. The Sea-lilies are so abundant in the Carboniferous 

 rocks, that it has been proposed to call the earlier portion of 

 the period the "Age of Crinoids." Vast masses of the lime- 

 stones of the period are " crinoidal," being more or less ex- 

 tensively composed of the broken columns, and detached plates 

 and joints of Sea-lilies, whilst perfect " heads " may be exceed- 

 ingly rare and difficult to procure. In North America the re- 

 mains of Crinoids are even more abundant at this horizon than 

 in Britain, and the specimens found seem to be commonly 

 more perfect. The commonest of the Carboniferous Crinoids 

 belong to the genera Cyathocrinus t Actinocrinus, Platycrinus, 



Fig. 117. Platycrinus tricontadactylus, Lower Carboniferous. The left-hand figure 

 shows the calyx, arm-', and upper part of the stem ; and the figure next this shows the sur- 

 face of one of the joints of the column. The right-hand figure shows the proboscis. (After 

 M'Coy.) 



(fig. 117), Poteriocrinus, Zeacrinus, and Forbesiocrinus. Closely 

 allied to the Crinoids, or forming a kind of transition between 



