134 
NATURAL HISTORY. 
[east. ZOOL. 
The family of Helicinas ( Helicinidce ) 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 reflexed edge ; they use 
their elongated tapering tentacles to feel their way as 
they walk. The Helictnce 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 
JLucidelice are peculiar among operculated shells for having 
three or four teeth on the thickened edge of their mouths. 
The second class of Mollusca, or Conchifera, 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 being placed (guarded by tw-o 
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 circulating 
within the cavity of the mantle for this purpose and 
that of supplying w r ater 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, 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 ahvays 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- 
