310 



PALAEOZOIC SYSTEM OF ROCKS. 



At present, leaving out the Holothurians, or sea-cucumbers, which, 

 having no shell, are little apt to be preserved as fossils, the class of 



Fig. 321. 



Fig. 322. 



Figs. 321 and 322.— Silurian Polyzoa: 321. Fenestella elegans (after Hall). 322. Alecto aulopo- 



roid.es (after Hall). 



Echinoderms may be conveniently divided into three orders, viz. : the 

 Echinoids, or sea-urchins ; the Asteroids, or star-fishes; and the Cri- 

 noids. The members of the 

 first and second orders are free 

 moving, while those of the 

 third are stemmed. Of these 

 orders the Crinoids are the 

 lowest, as proved not only by 

 their simpler organization, 

 but also by the fact that a 

 living Crinoid, the Comatula 

 (Fig. 325), is attached when 

 young, but free when mature. 

 Now, in Silurian times, 

 the stemmed Echinoderms are 

 very abundant, while the free 

 are very rare : at the present 



Fig. 323. Fig. 324. . J r 



Figs. 323 and 324.— Living Crinoids: 323. Rhizocrinus time, On the Contrary, the re- 



CapuSdi a " er Thompson) - 324 ' Pentacrhllls verse is the case. Thus, in 



the course of time, the former 

 decreased until they are now almost extinct, while the latter increased 

 until they are now very abundant. If we take the abundance of Echino- 

 derms during geological times as constant, and represent the course of 

 time by the absciss A B (Fig. 326), and the abundance by distance 

 from A B to C D, then the parallelogram would represent this fact. 



