m 
NATURAL HISTORY. 
[EAST. ZOOL, 
by extending its foot, attaches the dilated end of the fibre to some 
marine body, and then allows it to be withdrawn from the groove. 
New fibres are formed as they are required either by the breaking of 
the old ones or by the enlarged size and greater strength of the animal. 
The family of Tred acnid^s have the solid opaque white shell, and the 
broad and subquadrate foot of the preceding 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 oblique interlocking 
teeth. They are the giants among the Conchifera , 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 or coral to which they may 
happen to be fixed. The Tridacna is an ovate shell with a gape in the 
anterior slope for the passage of the byssus. The Hippopus , or horse- 
hoof shell is flattened, and closed in front. 
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 Mytilecle have the mantle-lobes free, with only a 
single distinct slit for the exit of the water and fseces; the shell is ovate 
triangular, with a marginal cartilage and sometimes one or two indistinct 
teeth under the umbo. In Mytilus the umbo is acute at the anterior 
margin of the shell, and in Modiolus it is placed rather behind the ex¬ 
tremity. 
The family of Crenelledjs 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 Crenelles are sub-orbicular, and the Modiolarice 
ovate elongated shells. * 
The family of Dreissenecee have been confounded with the Mytilidxz 
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. 
They are fluviatile. The Dreissence are triangular shells lengthened 
behind, like the Mytili ; the Congeria, on the contrary, are short, sub- 
quadrate, thick shells, truncated in front; the latter are only known as 
fossils. 
In the remainder the mantle-lobes are free all round without any par¬ 
ticular apertures, and the hinder end is bearded. They differ from the 
Micropoda chiefly in having two more or less unequal adductor muscles. 
The family of Pinned .E have a large triangular shell of a prismatic 
crystalline texture, united by a linear marginal cartilage, the apical part 
of the valves is divided in half by a central longitudinal suture filled with 
a cartilaginous 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 
