GALLERY.] NATURAL HISTORY. 117 
some have a simple round mouth* with a concave inner lip, 
as the Liltorince and Assiminia, Lacuna differs in the pillar 
lip being flattened, and the axis pierced; Scalaria is re¬ 
markable for having a white shell, marked with numerous 
varices; the Turritellce are turreted, and the Melanice 
and Cerithice , which have also generally elongated turreted 
shells, usually have a more or less distinct groove or nick 
in the front of the mouth. Some, as the Melanopses , have 
a nick quite like the Buccina; but they are easily known 
from the marine zoophaga by their structure and mode of 
life, which is passed in fresh water; their apices are gene¬ 
rally eroded, and they are often covered with a dark olive 
periostraca. The Tristoma are peculiar for having a con¬ 
tracted mouth, and a tubular hole, like the Typhis , at the 
posterior angle of the outer lip. 
The family of Looping Snails ( Truncatellidce ) have an 
oval spiral operculum, and the same form of animal as the 
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 Mollusca, with the opercula formed of 
regular rings, but they differ from the Ampullariadce 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 Valve Shells [Valvatidce) are small duvia- 
tile Ivlollusca, 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 family of Pyramid Shells (Pyramidellidce) are at once 
known by the oblique plaits on the front of the pillar lip 
of the shell, and by their tentacles 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 
