134 NATURAL HISTORY. [EAST. ZOQL. 
from the ligament, and placed in a triangular pit in front 
of the cardinal teeth. Their anterior hinge tooth is bifid, 
or triangular. 
The Mesodesmidce only differ from the former in the 
anterior cardinal tooth being single and compressed. 
The family of Tellinidcje have very long slender sy¬ 
phons, which are separated from each other in their whole 
length ; their shells are consequently marked with a very 
large syphonal inflection. The genus Donax is peculiar 
for having the hinder side of the shell much shorter than 
the front, giving it a truncated wedge-shaped form, which 
has caused most conchologists to mistake the front for 
the back of these shells. The Amphidesmcz only differ 
from the Tellince in having the cartilage placed in an in¬ 
ternal pit. 
The order of Clabopoda, on the contrary, have a large 
club-shaped foot, which is sometimes truncated and ex¬ 
panded at the end. The mantle-lobes are generally 
united together, forming a bag open only in front to allow 
of the passage of the foot. The syphons are large, 
exserted, and generally united together nearly to the end, 
and the gills are produced into the upper or anal syphon. 
The periostraca is extended so as to cover the exposed 
part of the mantle and syphons. 
The family of Pholadce have the valves united only by 
a very thin ligament, scarcely thicker than the periostraca, 
in which are generally imbedded one or more shelly 
plates. The place of the cartilage is supplied by a mus¬ 
cle situated on a reflection of the inner surface of the shell, 
over the umbones. They live in holes, which they form 
in the substance of shells, calcareous rock, wood, and resin. 
The foot is truncated in front, to enable them to retain 
their position while they rotate their shell to form the holes, 
the substance in which they burrow being partially softened 
beforehand by the juices of the animal. The Teredo , in¬ 
stead of having any plates in the dorsal ligament, continues 
to bore deeper and deeper into the w T ood, and lines; the 
holes as he proceeds with a shelly tube for his protection. 
The family of Gastrochcenad(S , have animals very similar 
to the former ; the valves are thin, gaping in front, and 
united by a narrow ligament with a thin marginal carti¬ 
lage. They live inclosed or sometimes imbedded in the 
