107 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
operculum; and the Lucidellce are peculiar among oper- 
culated shells for having three or four teeth on the thick¬ 
ened edge of their mouths. 
The second class of Bivalve Mollusc a 3 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 branchiae 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 surface. They have no 
distinct head, the mouth beingplaced (guarded by two pair 
of elongate fleshy lips, somewhat like the gills in appear¬ 
ance) 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 circulating 
within the cavity of the mantle for this purpose and 
that of supplying 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 faecal matter, at the 
dorsal angle of the same extremity, and the various modi¬ 
fications which this end of the mantle assumes, (being 
sometimes produced into syphons, at others simply 
united, leaving two holes, or quite free, to offer more 
or less facility to the entrance and exit of the cur¬ 
rent,) afford some of the best characteristics hitherto ob¬ 
served 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 perpendicular fibres ex¬ 
tended from the edge of one valve to the other, and is 
often so closely united to the inner surface of the liga¬ 
ment that they have generally been confounded together 
and regarded as one body; but their use is extremely dif¬ 
ferent, the former being to keep the valves together, and 
with the assistance of the teeth in their proper situations 
with respect to each other, while the use of the cartilage is 
