THE CARBONIFEROUS PERIOD. 



177 



these grooves there seems to have been attached a row of 

 short jointed calcareous filaments or " pinnules." 



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 mouth in the centre of the 

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

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



I 



r^o V 



°l 



>^ < 



°o) 



'° J 



' — ^ 



•^^ 





°\' 





0^1 



,0° 



) 2J 



'^^^ 



' '0) 



<o° 



^--d 



Fig. 119. — Palcechinus ellipticus, one of the Carboniferous Sea-urchins. The left- 

 hand figure shows one of the " ambulacral areas" enlarged, exhibiting the perforated 

 plates. The right-hand figure exhibits a single plate from one of the " inter-ambulacral 

 areas." (After M'Coy.) 



are arranged in ten zones which generally radiate from the 

 summit to the centre of the base. In five of these zones — 

 termed the " ambulacral areas " — the plates are perforated by 

 minute apertures or "pores," through which the animal can 

 protrude the little water-tubes (" tube-feet") by which its loco- 

 motion is carried on. In the other five zones — the so-called 

 " inter-ambulacral areas " — the plates are of larger size, and 

 are not perforated by any apertures. In all the modern Sea- 

 urchins each of these ten zones, whether perforate or imper- 

 forate, is composed of two rows of plates ; and there are thus 

 twenty rows of plates in all. In the Palaeozoic Sea-urchins, on 

 the other hand, the ''ambulacral areas" are often like those 

 of recent forms, in consisting of two rows of perforated plates 

 (fig. 119); but the "inter-ambulacral areas" are always quite 



