i8o 



HISTORICAL PALAEONTOLOGY. 



transversely-striated areas (fig. 118, C), each with a longitudi- 

 nal groove down its middle; and along each side of each of 

 these grooves there seems to have been attached a row of 

 short jointed calcareous filaments or " pinnules. " 



Fig. 118. A, Pentremitea pyriformia, side-view of the body ("calyx "); The same 

 viewed from below, showing the arrangement of the plates ; C, Body of Pentremites 

 conoideus, viewed from above. Carboniferous. 



A few Star-fishes and Brittle-stars are known to occur in the 

 Carboniferous rocks ; but the only other Echinoderms of this 

 period which need be noticed are the Sea-urchins (Echinoids). 

 Detached plates and spines of these are far from rare in the 

 Carboniferous deposits ; but anything like perfect specimens 

 are exceedingly scarce. The Carboniferous Sea-urchins agree 

 with those of the present day in having the body enclosed in 

 a shell, formed by an enormous number of calcareous plates 

 articulated together. The shell may be regarded as, typically, 

 nearly spherical in shape, with the mo*uth in the center of the 

 base, and the excretory opening or vent at the summit. In both 

 the ancient forms and the recent ones, the plates of the shell 

 are arranged in ten zones which generally radiate from the 

 summit to the center of the base. In five of these zones 

 termed the " ambulacral areas " the plates are perforated by 



