90 
NATURAL HISTORY. [NEW BUILDING. 
garded 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 
cylindrical, with a very small base, and the foot is clavate 
and not used for walking. 
The family of Lottladce , (Case 18,) the shells of which 
are so similar to those of the Patellce , that it is impossible 
to distinguish them from each other: the animals, how¬ 
ever, which form them are quite different from those of the 
latter shells, whilst they are closely allied to the Fissurellce , 
from which they scarcely differ, except in having only one 
gill, placed obliquely across the back, which is exserted 
when the animal walks. 
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 always opaque and porcellaneous. 
The tentacles are generally elongate and slender. 
The family of Nerites ( Neritidce , Case 18). 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 Ampnllariadce (Case 19) differ from all 
the foregoing in the operculum being annular; The shells 
are globular, ajid are covered with a thick olive periostra- 
cum. The animals have long filiform tentacula, a forked 
forehead, and their gills, which consist of only a single 
series of plates, is placed *n a cavity divided into two parts 
by a ridge, and are oviparous. The eggs are large, globular, 
greenish, and translucent, attached to plants under water. 
They live in fresh water. In some the operculum 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 Ianthinidce , (Case 19,) 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 this order. 
These animals, which generally float on the surface of the 
