. 
298 DIMYARIA.—PHOLADIDZ. 
such particles, though inclined to regard such a 
view with favour, as in this case the surface of 
the shell does not seem devised for rasping, as is 
that of the shells of the Pholadide.” * 
FamiILty PHOLADIDA. 
(Borers.) 
The members of this family have a very close 
affinity with those of the last, both in structure 
and habit, and have indeed been often united with 
them by zoologists. They possess peculiarities, 
however, which make their separation more natural. 
The genera differ among themselves more in out- 
ward appearance than in organization; they have 
the body usually lengthened in various degrees, 
and produced behind into very long siphons, which 
are united through their length. The mantle is 
closed except in front, where the foot protrudes ; 
this organ is club-shaped, and has the extremity 
obliquely flattened, so as to form a broad disk; it 
is sometimes largely developed, at others minute. 
The valves are equal, but very one-sided, always 
gaping at both extremities; the hinge is without 
teeth, and there is no proper hgament; each valve 
is furnished with a curved calcareous process 
beneath its summit, and at the back of the shell 
there are generally two accessory plates resemb- 
ling valves. 
The mollusks of this group are as indomitable 
borers as the Gastrochenade, but they less ex- 
clusively confine themselves to hard substances. 
* Brit. Moll. i. 138. 
