78 
NATURAL HISTORY. 
[EAST. ZOOL, 
edges of the mantle-lobes are thickened, and the syphons are elongate 
and more or less united; the shells are oblong, thin, and brittle, 
covered with a granular or spinulose periostraca ; the hinge is obscurely 
toothed; the cartilages are internal, placed in a pit in each valve, and 
furnished with a peculiar shelly hinge-piece, which is either placed 
before it or sometimes so as to cover the whole of its surface. In 
most of the genera the shell is regular and free. In Auriscalpium , 
Periploma , Cochlodesma and Hemicyclostoma, the cartilages are in a 
pit on a projecting spoon-shaped process, and the hinge piece is small. 
In Thracia the cartilage is on a callosity of the hinge margin. In the 
Lyonsice and Myadorce , the cartilage pit is sunk into the hinge margin 
of each valve, and covered by a large flat hinge piece; the shell of the 
former is thin and of the latter thick, with very unequal valves, the left 
one being flat. Camostrea and Myochama differ from the rest in the 
shell being irregular and attached by the outer surface of one of the 
valves. 
The family of Myadce are very like the former, but the cartilage is 
placed on a spoon-shaped cavity in one of the valves, fitting into a pit 
in the surface of the other; there is no hinge piece, and the hinge is 
toothless; the syphons are long and the lips small. In My ce the spoon¬ 
shaped projection is oblong ; in Sphcenia it has a process behind, which 
unites it with the hinder part of the hinge margin. The Tagonia have 
the same kind of process, but the shell is ovate, ventricose, with a large 
gape on its short hinder slope. 
The family of Corbulim: differ from the former in having small 
tubes and very large lips ; in the right valve being the largest; and in 
being provided with a large tooth in each valve in front of the cartilage 
pit; there is no syphonal inflection. The Corbulce are thick, nearly 
triangular shells, with a large hinge tooth in each valve. The Harlea 
are oblong, subquadrate, thin shells, with a sharp keel from the umbo, and 
conical hinge teeth. The Tomala are like the Corbulce, but have a 
triangular projecting plate with a ridge on each side in the left valve, 
and two triangular teeth in the other, and Raleta differ from the latter 
only in having a narrow central pit, its right valve with a strong (fonical 
tooth falling into the large pit before the tooth in the left valve. The 
Neara have a thin nearly equivalve shell produced into a beak behind, 
and with small hinge teeth. 
The family of Pandorhee have very unequal valves and pearly shells, 
with two diverging teeth in each valve, near the cartilage pit. They 
have tw r o short tubes, like the Corbulidce , but the foot is small, the gills 
on each side are united into one, and it is the left valve of the shell 
that is the most concave. 
The family of Solenomyad^e are very peculiar for being covered 
with a very hard cartilaginous periostraca, which is much produced 
beyond the edge of the shell; they have only one gill on each side, no 
lips, and the foot truncated and ciliated round the edge of the radiated 
grooved end, by means of which they very quickly bury themselves in 
the mud. They have only a single large ciliated syphon ; the hinge is 
toothless, and the cartilage linear, partly internal, with a slight pad on 
each side. 
The family of Galeomid^l have a squarish equal-valved shell, with 
a straight toothless hinge and a large ovate gape below, the mantle lobes 
