91 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
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 (Atalantidce , 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 second division of Phytophagous Ctenobranchous 
Mollusca , or Eriophthahni, 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 
generally regularly spiral, with a moderately-sized aperture. 
The family of Nipple Shells ( Naticidce , Case 19) are 
peculiar for having a large foot, in which the hemisphe¬ 
rical shell is imbedded, and which is much produced in 
front, beyond its edge; the tentacles are small, sometimes 
obliterated, and the mouth ishid in a groove; the operculum 
is spiral. 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 Natica and 
Nacca , which have a moderate mouth, have a large oper¬ 
culum ; and Cryptostoma , which has a very large mouth, 
has a very small operculum. 
The animals of the following families have usually a 
moderately sized foot. 
The family of Periwinkles (hittorimdee , Case 20) have 
