Ill 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
shell cordate and radiately ribbed, with two large teeth 
in each valve, placed in a cross, and distinct lateral teeth. 
The ligament is external. 
4. The family of Mactridce have the cartilage separated 
from the ligament, and placed in a triangular pit behind 
the cardinal teeth. Their anterior hinge tooth is bifid, 
or triangular. 
5. The Mesodesmidce only differ from the Mactridce in 
the anterior cardinal tooth being single and compressed. 
6. The family of Tellinidce 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 cartilage is generally ex¬ 
ternal. 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 Amphidesmce only differ from 
the Tellince in having the cartilage placed in an internal 
pit. 
The order of Cladopoda, 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, 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 
