88 
NATURAL HISTORY, [NEW BUILDING* 
and 0villa, 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. They are said to live on sea 
plants under rocks near the breakers; they walk rapidly, 
but expand and contract their mantle very slowly. 
Near this family must be placed, for the present, the 
anomalous genus Coriocella , (Case 16,) which is peculiar 
among these animals for having a thin, white, ear-]ike shell, 
imbedded in the large expanded coriaceous mantle, which 
is nicked in front for the syphon. 
2. The Phytophagous Gasteropodes , (Pliytophaga ,) so 
called because they live chiefly on vegetable food, are desti¬ 
tute of any distinct 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 deposited on the surface of other shells ; but many of 
the animals are ovoviviparous. Many of them have a spiral 
operculum 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 two sections 
according to the position of their eyes, as Podophthalmi | 
and Eriophthalmi , p. 91. , 
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 r 
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 Turbos ( Turbinidce , Case 16) 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 a 
leaf-like appendage at their bases. They are mostly covered 
with a thick periostraca, but some, as Phasianella, are 
