GALLERY.]] NATURAL HISTORY. I13 
the shell is gradually contracted and indexed, only leaving 
a narrow slit, and its back is covered with an enamel coat, 
deposited on its sides by the lobes of the mantle. In 
Cyprcea, Algoa , and Ovula , the outer coat of the shell is 
polished, but in Cyprovula and Trivia it is covered with 
raised ribs. In most of the genera both the lips are plaited, 
but in Ovula the inner one is smooth. 
Near this family must be placed, for the present, the 
anomalous genus Coriocella , (Case ,) which is peculiar 
among these animals for having a thin, white, ear-like shell, 
imbedded in the large expanded coriaceous mantle, which 
is nicked in front for the syphon. 
2. The Phytophagous Gasteropodes , so called because 
they live chiefly on vegetable food, are destitute of any dis¬ 
tinct syphon for the passage of water to the branchial 
cavity, and therefore have no canal in front of the mouth 
of the shell; their eggs are membranaceous, and often depo¬ 
sited on the surface of other shells ; but many of the animals 
are ovoviviparous. Many of them have a spiral oper¬ 
culum or lid, which is attached to the back of the hinder 
part of the foot of the animal: this operculum turns round 
backwards on the apex of its spire, as it increases in size, 
by the addition of new matter to the edge of its last whorl, 
so that this edge is always in the same position in the 
mouth of the shell. They are divided into sections accord¬ 
ing to the position of their eyes. 
The Podophthalmi have their eyes placed on short pedi¬ 
cels at the. back inner angle of the tentacles; their heart 
generally surrounds the rectum. 
In most the sides of the body are furnished with a fringe 
sending out a series of filaments or tentacles. These are 
all marine, often living on rocks near the shore; when left 
by the tide they shut a quantity of fluid into the respira¬ 
tory cavity, and close the mouth of the shell with the oper¬ 
culum to prevent its evaporation until the water again 
covers them. Many of them are hermaphrodite, like the 
snails. 
The family of Turbinidce (Case ) have solid spiral 
shells, with a roundish aperture, which is generally pearly 
within. Their operculum is spiral, and protected by a 
hard external shelly deposit; and the tentacles have an 
appendage at their bases. They are mostly covered 
