144 NATURAL HISTORY. [EAST. ZOOL. 
The rest of the animals of this order have a tongue¬ 
shaped foot, their shells are generally of a prismatic crys¬ 
talline texture, and the hinder adductor muscles are much 
larger than the front. 
In some of these the mantle-lobes are more or less 
united together. 
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 
bearded. They differ from the Macropoda chiefly in 
having two more or less unequal adductor musbles. 
The family of Pinnidce have a large triangular shell of 
a prismatic crystalline texture, united by a linear mar¬ 
ginal cartilage, the apical part of the valves is divided in 
half by a central longitudinal suture filled with a cartila¬ 
ginous substance. The animal has two double lips besides 
the usual pair of appendages, and by the side of the vent, 
which is above the large hinder muscle, there is a conical 
contractile appendage, the use of which is unknown. 
They live sunk in the sand, or between cracks in rocks, 
with their gaping truncated end just above the surface. 
The beards of these animals are sometimes spun into 
gloves, &c., like silk. 
The family of Aviculidce differ from the former in 
having a notch in the front margin of the right valves for 
