NATURAL HISTORY. 
115 
GALLERY.^ 
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, Dentalium . 
The family of Lottiadce , the shells of which are so simi¬ 
lar to those of the Patella , 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 Fissurelite, 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 Podophthalmi are destitute of any 
appendages or tentacles 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 ( Neritida , 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 , Neritina, 
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 Ampullariadce (Case ) differ from all 
the foregoing in the operculum being annular; the shells 
are globular, and are covered with a thick olive periostra- 
cum. The animals have long filiform tentacula, a forked 
forehead, and are oviparous. The eggs are large, globular, 
greenish, and translucent, attached to plants under water. 
They live in fresh water. In some the ojierculum is 
simply horny ; others, which have a thick edge to the 
mouth of the shell, have a thick shelly internal coat to the 
operculum. 
The family of Violet Shells, or lanihinidce , (Case ,) so 
called from the fine blue colour of the shells, which appear 
to be stained by the abundant violet juices of the animals, 
seem to be most nearly allied to the last mentioned family. 
These animals, which generally float on the surface of the 
sea, have a large head and a small oblong foot, which has a 
mass of cartilaginous bubbles attached to the middle of the 
ventral surface, serving the office of a float, and on which 
