EAST. ZOOL. GAL.] NATURAL HISTORY. 117 
shell, and the broad and subquadrate foot of the preced¬ 
ing families ; but they are very peculiar for having the 
mantle-lobes united, so as to leave three apertures, 
and for the animal being so placed in the shell that the 
hinder adductor muscle is in the middle of the lower 
margin, between the two syphons, and the hole through 
which the foot and byssus are passed out high up and 
near the umbo, where the gap is between the front 
edges of the valves ; the hinge is furnished with very ob¬ 
lique interlocking teeth. They are the giants among the 
Cone infer a, and live attached by their byssus to rocks, 
shells, and corals. They also have the faculty of forming 
holes in the surface of the shell to which they may happen 
to be fixed. 
The rest of the animals of the families of this order 
have a tongue-shaped foot, their shells are generally of 
a prismatic crystalline texture, and the hinder adductor 
muscles are much larger than the front. 
In some of these the mantle-lobes are more or less 
united. 
The family of Mytilidce have the mantle lobes free, 
with only a distinct anal slit; the shell is ovate triangular, 
with a marginal cartilage, and sometimes one or two in¬ 
distinct teeth under the umbo. In Mytilus the umbo is 
acute at the anterior margin of the shell, and in Mo¬ 
diolus it is placed rather behind the extremity. 
The family of Crenellidce chiefly differ from the former 
in the mantle-lobes being united together so as to leave 
only two posterior holes for the entrance and exit of 
the water, and a slit for the foot and beard; the hinge 
margin is denticulated on each end, and the umbo is 
nearly central. 
The family of Dreissenidce have been confounded with 
the Mytilidce like the last family, with which they agree 
in having the mantle-lobes united ; but they differ from 
them in having a large, elongated, conical, inferior syphon: 
the umbo is acute, and placed at the front apex of the 
shell, as in Mytilus, and it has a septum forming a cavity 
beneath it. 
In the remainder the mantle-lobes are free all round 
without any particular apertures, and the hinder end is 
