FOSSIL GASTEROPODA. 419 
condition of its margin, and the presence or absence of a 
canal or siphon always having relation to the soft parts of 
the animal, these characters afford data by which the genera 
and species of the shells may be determined, and informa- 
tion obtained as to the structure and economy of the 
originals, 
The Gasteropoda generally creep by means of a fleshy 
disk, or foot, which is situated under the belly. Some kinds 
are terrestrial, others inhabit trees, many live in rivers and 
streams, others in stagnant and brackish waters; but the 
greater number are denizens of the sea. 
The Common Snail, River Snail, and Periwinkle, are 
instances of terrestrial, fluviatile, and marine forms. The 
organs of respiration are situated in the last whorl of the 
shell; and in some genera the border of the mantle, or 
integument surrounding the body, is prolonged into a siphon, 
by which the water is freely admitted, without the head or 
foot being protruded : in these mollusks the shell has a cor- 
responding channel to receive the siphon, as in the Whelk, 
or Buccinum, and in the fossil shell Lign. 133, fig.4. The 
Gasteropoda are generally provided with an operculum, or 
moveable valve, by which the aperture is closed and defended 
when the animal retreats within its shell. In some species 
the operculum is a mere horny pellicle; in others it isa 
solid calcareous plate of considerable relative thickness. 
These mollusca, as is but too well known of the terrestrial 
species, consume large quantities of food. Some are herbi- 
vorous, and others .carnivorous; many prey on living, and 
others on decaying animal and vegetable substances.* As 
* “ All Gasteropoda commence life under the same form, both of 
shell and animal, namely, a very simple helicoid shell, and an animal 
furnished with two ciliated wings or lobes, by which it can swim 
freely through the fluid in which it is contained. At this stage of 
existence the animal corresponds to the permanent state of the 
Pteropod, and the form is alike, whether it be afterwards a shelled or 
