MOLLUSCA. 77 



vours to prove by very inconclusive reasoning, that these shells 

 are formed posterior to the birth of the animal. He supposes 

 that, when they become too small to contain the inhabitants on 

 account of their increasing size, the old shells are forsaken, and 

 more commodious dwellings formed, until the animal reaches 

 its full size. Other proof, however, than what the author ad- 

 duces, is necessary to render the opinion probable. 



Lamark, in his Systeme des Animaux sans vertebres, 

 placed these shells in a separate section at the end of the 

 bivalves, and among the acephalous mollusca. Afterwards 

 he considered them as constituting a particular division of 

 the crustaceous animals; and, lastly, he has assigned them 

 a place in his new class, which he terms CIRRHIPIDES. 



3. PHOLAS. This very natural genus was placed among 

 the multivalves by Linnaeus, in the twelfth edition of his 

 system. It is now united with the bivalves, the accessory 

 plates at the hinge being regarded as of subordinate import- 

 ance. In other respects it has stood the test of modern in- 

 novation, with the exception of the genus GASTROCIOSNA 

 of Spengler, in which the teeth are obsolete. This includes 

 the pholas hians of Chemnitz, and the mya dubia of Pen- 

 nant. 



4. MYA If we consider as definite the character as- 

 signed to this genus by Linnaeus himself, we shall find that 

 it excludes many species which differ from the M. truncata> 

 at present considered as the type of the genus. In this 

 shell, the valves gape at both extremities, the ligament is 

 internal, and placed on a thick erect tooth in one valve,jnot 

 inserted into the opposite side. As the M. vulsella of Lin- 

 naeus is close at both ends, and destitute of a tooth, it has 

 been separated from the true myae, and formed into a dis- 

 tinct, genus by Lamark, under the title VULSELLA. This 



