THE CARBONIFEROUS PERIOD. r73 
disappeared, and it is never again represented save sporadi- 
cally and by isolated forms. 
Amongst the “chinoderms, by far the most important forms 
are the Sea-liles 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, Actinocrinus, Platycrinus, 
Fig. 117.—Platycrinus tricontadactylus, Lower Carboniferous. The left-hand figure 
shows the calyx, arms, 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 Forbestocrinus. Closely 
allied to the Crinoids, or forming a kind of transition between 
