118 
NATURAL HISTORY. [EAST. ZOOL. 
the foregoing in the operculum being annular; the shells 
are globular, and are covered with a thick olive periostra- 
cum. The animals have long filiform tentacula, a forked 
forehead, and their gills, which consist of only a single 
series of plates, is placed in a cavity divided into two parts 
by a ridge, and are oviparous. The eggs are large, globular, 
greenish, and translucent, attached to plants under water. 
They live in fresh water. In some the operculum is 
simply horny ; others, which have a thick edge to the 
mouth of the shell, have a thick shelly internal coat to the 
operculum. 
The family of Violet Shells, or Ia?ithinidce, (Case ,) 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 whorls have 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 (Atplantidce ) have the same 
habits, and much of the appearance of the former animals, 
but the foot is smaller, and the middle of its ventral sur¬ 
face is provided with an erect compressed rounded fin, with 
a sucking disk on its hinder edge, and there is af 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 second division of Phytophagous Ctenobranchous 
Mollusca , or Eriophihalmi, 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 trian¬ 
gular plates, and are not exposed; and their shells are 
