46 PICTURESQUE SKETCHES 



of crinoids (lily-shaped polyps), individually abun- 

 dant ; and although there appear to be one or two 

 species of the more highly organized Radiata (such as 

 the star-fish), we very rarely find remains of other radi- 

 ated animals than encrinites in the old rocks, although 

 numerous higher forms afterwards became exceed- 

 ingly abundant. Of the crustaceans again we obtain 

 no fragments of true crabs or lobsters, or other com- 

 mon and known forms, but instead of them a group, 

 long since extinct, not more likely to be preserved 

 than the former, and, although for a time evidently 

 very common, not continued into the middle one of 

 the three great periods. 



The absence or rarity of the common bivalve and 

 univalve shells in these rocks is also a point of very 

 considerable interest. A few species of the family 

 represented by the common cockle (Cardium), and a 

 few also of the scallop tribe (Pecten, Avicula), both of 

 which groups are remarkable among the shell-bearing 

 animals for their locomotive powers, and the extent 

 to which they adapt themselves to changing cir- 

 cumstances, are among the chief of the bivalves ; and 

 a number of species nearly allied to the carnivorous 

 Buccinum, or whelk, represented in like manner the 

 univalves. But although the genera now common 

 were then so rare, their place was evidently supplied 

 by two other groups now nearly lost sight of; and of 

 these the vast number of shells allied to Terebratula, 

 and the abundance of Orthoceratites, form the most 

 striking and valuable examples. 



Quite at the close of the period we are considering, 

 a few small fishes, apparently allied to the shark 

 tribe, were also introduced as typical forms of what 



