MOLLUSCA. vrs 
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 oldshells 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 Systéme 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 CrrRHIPIDEs. 
3. Puotas.—This very natural genus was placed among 
the multivalves by Linneeus, 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 GASTROCHANA 
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 Linneus 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, not 
inserted into the opposite side. As the M. vulsella of Lin- 
nzeus is close at both ends, and destitute of a tooth, it has 
been separated from the true myz, and formed into a dis- 
tinct genus by Lamark, under the title VunseLia. This 
