120 
NATURAL HISTORY. [EAST. ZOOL. 
former family, but the foot is small, and the lips very large 
and expanded ; the animal walks by contracting the space 
between these parts into a loop, like the Looper or Geometric 
Caterpillars. 
The family of Pond Snails, ( Paludinidce ,) like the Am- 
pullariadce , and unlike the other families of this group, 
are fresh water Moilusca, with the opercula formed of 
regular rings, but they differ from the Ampullarladce in 
having short tentacles, sessile eyes, and an entire forehead. 
The Paludince have a horny operculum, and are ovovivipa- 
rous, while the Bithinice , which have a shelly operculum, 
are oviparous like Lymncea. 
The family of Velutinidoe have an animal very like the 
Littorinidce, but they are destitute of any operculum, and 
the shell is small for the size of the animal, with a very 
short spire and a large mouth. They live on stones on 
the shore. 
The family of Pyramid Shells {Byramidellidce) are at once 
known by the oblique plaits on the front of the pillar lip 
of the polished, spotted, turreted shell, and by their tenta¬ 
cles being broad and folded like the ears of a hare; they 
have the eyes at the inner side of their base, like the 
Auriculce; their operculum is horny, with a flap on the 
inner side to adapt it to the plaits on the pillar. 
The family of Turned Shells ( Tornatellidce ) also have 
plaits on the pillar, and a horny operculum with a flap; 
but the animal has no true tentacles, only an expanded disk 
on the front of the body, somewhat like the disk of the 
Bulladce . * 
2. The animals of the families which follow have their 
gills formed of long filaments, and often exserted when the 
animal is expanded; their shells are very variable and 
anomalous in form, and often have a very large aperture. 
The family of Valve Shells (Falvatidce) are small fluvia- 
tile Moilusca, found in rivulets, with an orbicular many- 
whorled spiral operculum ; they are remarkable for having 
their gills, which are formed of spiral plates, protruded 
beyond the shell when they walk. The shells are regular 
spiral, with a round simple mantle, and covered with a 
pale olive periostraca. 
The family of Worm Shells ( Vermetidce ) are peculiar 
for having an irregularly tubular shell, which is generally 
