116 
NATURAL HISTORY. [EAST. ZOOL. 
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 ) 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 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 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 trian¬ 
gular plates, and are not exposed; and their shells' are 
generally regularly spiral, with a moderately-sized aper¬ 
ture. 
The family of Nipple Shells ( Naticidce) are peculiar for 
having a large foot, in which the hemispherical shell is im¬ 
bedded, 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. 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 cGral, under the name of Flustra are - 
nosa . The genera Natica and Nacca , which have a 
moderate mouth, have a large operculum; and Crypto- 
stoma, which has a very large mouth, has a very small oper¬ 
culum. 
The animals of the following families have usually a 
moderately sized foot. 
The family of Periwinkles ( Littorinldcc ) have a free 
oval, spiral, or subspiral horny operculum ; the shells ge¬ 
nerally have a roundish mouth, and are not pearly within; 
