II 



THE LAMELLIBRANCHIA 



ASSOCIATED with the terebratuloids in the Cornbrash, 

 though for the most part less well preserved, are other 

 bivalve shells obviously different from brachiopods. 

 In many of them the two valves are obviously right 

 and left, not dorsal and ventral, since they are like 

 mirror - images of one another ; and though in some 

 the two valves are unequal, they do not show the 

 perfect symmetry of brachiopod-valves. There is never 

 a pedicle-perforation, and though there is a hinge-line 

 and often a cardinal area not unlike those of brachiopods, 

 the internal muscle-impressions are very differently 

 arranged, and there is never a brachial skeleton. 



These bivalves belong to a distinct phylum from the 

 Brachiopoda the great phylum Mollusca, of which they 

 constitute a class, the Lamellibranchia. The resemblances 

 between them and brachiopods is due to their leading a 

 similar life and having the same needs ; the differences 

 are due to the fact that they had a different ancestry, so 

 that a different initial structure had to be adapted to the 

 same needs. 



As in brachiopods, each valve has an umboand near it 

 the hinge-line, but the anatomy of the animal shows that 



45 



