58 
NATURAL HISTORY. 
[EAST. ZOOL. 
absorbed as the animal grows, and the hole is furnished with a funnel- 
shaped appendix within the cavity of the shell. Macrochisma has a very 
large hole nearly extending to the back edge of the shell. In Pupillia 
the shell is surrounded by a sharp white edge; and in Lucapina the 
mantle covers the cancellated shell, which has a large hole and a toothed 
edge. 
The family of the Tooth-shells (Dentaliam:, Case 18) must also 
be placed here; they have been, till lately, regarded as the tubes of worms, 
but are now known to be formed by true Mollusca; they chiefly differ 
from the former family in the back being much higher and nearly cy¬ 
lindrical, with a very small base, and the foot is club-shaped and not 
used for walking. The true tooth-shells have a simple apex ; the Entalis 
has a slit in the front edge of the apex, and when the apex is accidentally 
destroyed, the animal often secretes a spoon-shaped appendix, exserted 
from the apex of the cavity. 
The family of Lottiad^e, (Case 18,) the shells of which are so simi¬ 
lar to those of the Patella, that it is impossible to distinguish them from 
each other ; the animals, however, which form them are quite different 
from those of the latter shells, whilst they are closely allied to the Fis- 
surellce, from which they scarcely differ, except in having only one gill, 
placed obliquely across the back, which is exserted when the animal 
walks. Like the Patella , the shell is modified in shape by the situation 
they happen to be attached to. This and the great variety produced by 
the abundance or scarcity of food, and by the stillness or roughness of 
the sea in the place they inhabit, makes it very difficult to distinguish 
the species of this genus. 
The remainder of the Podophthalmi are destitute of any appendages 
or beards on the sides of the feet, and the insides of their shells are al¬ 
ways opaque and porcellaneous. The tentacles are generally elongate 
and slender. 
The family of Nerites (Neritid^e, Case 18) have semiovate shells, 
with a small semicircular mouth furnished with a sharp transverse 
inner lip ; they have the operculum articulated to the pillar lip, as the 
genera Nerita, Neritina, and Navicella ; the former, which are/narine, 
have a shelly operculum grooved on the edge, and the two latter, found 
in fresh water, a thin operculum with a flexible margin. The eggs of 
some, as the Nerita, are ovate, covered with a horny skin, and attached 
to other shells. The Pileoli are (fossil) shells of a conical form with 
a circular base; the inner lip is expanded as far back as the hinder 
edge of the whorls, forming an edge to the base. The Culana differ 
in being oblong and rather convex beneath. The crown nerites ( Cli- 
thon ) chiefly differ from the Neritina in having a tubular appendage 
on the back edge of the outer lip, which forms a series of spines round 
the spire. The Postice differ in having a nearly symmetrical shell 
with only the rudiment of a spire, and the inner lip, like Neritina, is 
only denticulated. The Velates have the inner lip very thick and 
callous behind; they are only known as fossils. The Navicella, or 
slipper-shell, has a nearly symmetrical shell with a simply recurved tip 
and a very large mouth, having much the appearance of some long 
Patella . 
The family of Ampullariad^ (Case 19) differ from all the fore¬ 
going in the operculum being annular; the shells are globular, and are 
