GALLERY.] NATURAL HISTORY. 117 
formed by the growth of the animal, the last being occu¬ 
pied by the vent; and in Scissurella and Pleurotomaria 
there is a long slit in the shell, over the opening in the 
mantle, 
The family of the Keyhole Limpets ( Emcirgimdidce , Case 
) have an animal very like the Ear-shells, but the shell is 
depressed, simply conical, not pearly, and furnished either 
with a hole, placed in the front of the apex, as in Fis- 
surella , or with a notch in the front of the edge, as in 
Emarginiila and Parmophoras. These holes or grooves 
afford a passage for the water to the respiratory organs, 
and for the expulsion of the faeces. In Pitpillia the shell is 
surrounded by a sharp white edge; and in Lucapina the 
mantle covers the cancellated shell. 
Here must also be placed the family of the Tooth-shells, 
(Z) entaliadoe, Case ,) which have been, till lately, re¬ 
garded as the tubes of worms, but are now known to be 
formed by true Mollusca; they chiefly differ from the 
former family in being much higher and nearly cylindrical, 
with a very small base, and the foot is clavate and not used 
for walking, as the Tooth-shell, Fentalium . 
The family of Lottladce, the shells of which are so simi¬ 
lar to those of the Patellce, that it is impossible to distin¬ 
guish 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 Fissur elite, from which 
they scarcely differ, except in having only one gill, placed 
obliquely across the back, which is exserted when the ani¬ 
mal walks. 
The second group of the Podopkthalmi are destitute of any 
appendages or beards on the sides of the feet, and the 
insides of their shells are always opaque and porcellaneous. 
The tentacles are generally elongate and slender. 
The family of Nerites (Neritidce , Case ). Their shells 
are semi-ovate, 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, Nerltina , 
and Navicellus; the former has a shelly operculum grooved 
on the edge, and the two latter, a thin one with a flexible 
margin. The eggs of some, as the Neritce, are ovate, co¬ 
vered with a horny skin and attached to other shells. 
The family of Ampallariadce (Case ) differ from all 
