72 
NATURAL HISTORY. 
[east. ZOOL. 
which are of the size of the mouth of the shell, so that it at length 
assumes a cylindrical form and is partly supported by a shelly se¬ 
cretion. In others, as Megalomastoma , there is a permanent append¬ 
age to the mantle, which produces a groove and ridge in the front of 
the mouth near the pillar ; and in Pupina, which has been confounded 
with the Buccina , the groove ends in a narrow-edged marginal notch. 
The Cattice have a peculiarly polished shell very Tike the former, but 
they want the groove. The fossil genus Strophostoma is peculiar for 
the animal, when it is approaching to its adult state, suddenly reversing 
the position of its body, so that the mouth of the shell is placed on a 
plane with the surface of the spire, as in the genus Anostoma among 
the Helices. The Pomatias have an elongated shell with reflexed lips, 
and a horny oval operculum. 
The family of Helicinas ( Helicinidce , Case 26) have a half ovate 
annular operculum, and the edge of their mantle is said to be thickened 
like that of the snails. The mouth of the shell is half ovate, with a re¬ 
flexed edge; they use their elongated tapering tentacles to feel their 
way as they walk. The shell of the Helicince have a simple mouth. 
Alcadia differs in having a slit in front of the mouth, into which is fitted 
the tooth-like process of the operculum; and the Lucidellce are peculiar 
among operculated shells for having three or four teeth on the thick¬ 
ened edge of their mouths. 
The second class, or Bivalve Mollusca, or Conchifera, ( Cases 
27—37,) have the animals always covered with a two-lobed mantle, each 
protected by a shelly valve, and they have within the mantle, between it 
and the compressed body, a pair of laminar gills on each side. The lower 
part of the body is generally dilated into a keeled or horn-shaped foot, 
by which they walk along the sand or mud of the shore, or a flat disk, 
by which they attach themselves to rocks and form holes in their sur¬ 
face. They have no distinct head, the mouth being placed (guarded by- 
two pair of elongate fleshy lips, somewhat like the gills in appearance) 
at the back of the cavity between the mantle-lobes, near the front of 
the base of the foot. They depend for nourishment on the food which 
is brought near this aperture by the currents that are continually circu¬ 
lating within the cavity of the mantle for this purpose and that^of sup¬ 
plying water to the gills to aerate the blood, hence they are all aquatic. 
This current enters on the lower side of the hinder end of the mantle 
and shell, and makes its exit, carrying with it the fsecal matter, at the 
dorsal angle of the same extremity, and the various modifications which 
this end of the mantle assumes, (being sometimes produced into sy¬ 
phons, at others simply united, leaving two holes, or quite free, to offer 
more or less facility to the entrance and exit of the current,) afford some 
of the best characteristics hitherto observed to divide these animals into 
orders. The two valves, which are each formed and enlarged exactly 
after the plan of the shell of Gasteropodes, are always united together 
on their dorsal edges by a ligament of greater or less strength, and 
within this ligament there is placed an elastic cartilage, formed of per¬ 
pendicular fibres extended from the edge of one valve to the other, and 
is often so closely united to the inner surface of the ligament that they 
have generally been confounded together, and regarded as one body ; 
but their use is extremely different, the former being to keep the valves 
together, and with the assistance of the teeth in their proper situations 
