59 
^GALLERY.] NATURAL HISTORY. 
covered with a thick olive periostraca. The animals have long filiform 
tentacula, a forked forehead, and their gills, which consist of only a 
single series of plates, are placed in a cavity divided into two parts by a 
ridge ; they are oviparous. The eggs are large, globular, greenish, and 
translucent, attached to plants under water. They live in fresh water. 
The genus Ampullaria has a thick edge to the mouth of the shell, and 
a thick shelly internal coat to the operculum. Asolene chiefly differs 
from the former in the animal having no syphon to the gill cavity; the 
operculum is simply horny. In Marisca and Lanistes, the former has 
dextral and the latter sinistral shells. The Paludomi are thick, ovate, 
imperforated shells, with a thickened inner lip, which have been mistaken 
for Melania. 
The family of Violet-shells, or Ianthinid^e, (Case 19,) so called 
from the fine blue colour of the shells, which appear to be stained by 
the abundant violet juices of the animals, seem to be most nearly allied 
to this order. These animals, which generally float on the surface of 
the sea, have a large head and a small oblong foot, which has a mass of 
cartilaginous bubbles attached to the middle of the ventral surface, 
serving the office of a float, and on which the animals deposit their 
eggs. This part is probably a modification of the operculum. The 
shells are thin, with a large angular mouth, and the whorl has a deep 
notch in the middle of the outer lip, which is occupied by the neck of 
the animal when it is floating. 
The family of Atalants ( Atalanthle, Case 19) have the same habits, 
and much of the appearance of the former animals, but the foot is 
smaller, and the middle of its ventral surface is provided with an erect 
compressed rounded fin, with a sucking disk on its hinder edge, and 
there is a distinct shelly operculum on its peculiar mantle. Their shells 
are thin, transparent, and sometimes almost cartilaginous, with an 
angular mouth having a nick in the middle of the outer lip ; the whorls 
are often keeled. The Atalants have a top-shaped spiral shell when 
young, but it becomes discoidal and keeled as they increase in size. 
The Helicophora always have oblong spiral unkeeled shells with an 
entire mouth. 
The second division of Phytophagous Ctenobranchous MoUusca , dr 
Eriophthalmi , are so called because their eyes are sessile, or only placed 
on a very small prominence at the base of the tentacles ; their sides are 
simple. They are unisexual, and they are most usually provided with 
a distinctly spiral operculum. They are oviparous, but a few have their 
eggs hatched in the oviduct of the parent. 
1. In the following families the gills are formed of triangular plates, 
and are not exposed; and their shells are generally regularly spiral, 
with a moderately-sized aperture. 
The family of Nipple Shells (Naticidjs, Case 19) are peculiar for 
having a large foot, in which the hemispherical shell is imbedded, and 
which is much produced in front, beyond its edge ; the tentacles are 
small, sometimes obliterated, and the mouth is hid in a groove; the 
operculum is spiral. They are carnivorous, forming holes in and eating 
the animals of bivalve shells, which usually live in similar localities with 
them. The eggs are deposited in a broad, expanded band, folded like 
a funnel, sunk in the sand on the sea shore ; these bands have been 
described as a coral, under the name of Flustra arenosa. The genera 
