The typical gastropod has a spirally coiled, single-valved, unchambered
shell. This shell encloses a soft body possessing a well-defined head
with a pair of eyes and one or two pairs of tentacles. Most gastropods
have gills and live in shallow marine waters, but some inhabit fresh
water. Others are land-dwelling forms and breathe by means of lungs.
Gastropod shells, both Recent and fossil, exhibit a great variety of
size, shape, and ornamentation. Such shells may be cone-shaped, spirally
coiled, flat, turreted, or cylindrical. The shell is commonly wound in a
spiral around a central axial pillar (the _columella_). The closed
pointed end of the shell is called the _apex_, and each turn of the
shell is called a _whorl_ (fig. 16). The last-formed and largest whorl
is called the _body whorl_, and this whorl contains the _aperture_—the
opening of the shell. The combined whorls exclusive of the body whorl
are known as the _spire_. The inner and outer margins of the aperture
are designated the _inner lip_ and the _outer lip_, respectively. In
some snails the aperture is closed by means of the _operculum_—a
calcareous or horny plate attached to the foot of the animal. This plate
effectively seals the aperture when the animal is withdrawn into its
shell. Some gastropods have shells that are loosely coiled, and in these
forms the columella is absent. If the whorls of such shells are not in
contact on the inner surface, this leaves an open space which is called
the _umbilicus_ (fig. 16a). The umbilicus is commonly seen as an opening
in the base of the gastropod shell, but in some forms the umbilical
opening may be partially or completely covered by a thick growth of
shell called the _callus_.
Many gastropods, particularly those of the Texas Cretaceous, are
commonly preserved as internal or external molds. This type of
preservation occurs after the death of the animal, and the decomposition
of the soft parts enables the shell to become filled with sediment. This
filling later becomes solidified, and the outer shell may eventually be
removed by weathering or solution. This type of internal mold is called
a _steinkern_ and normally does not reflect any external shell
characteristics (Pl. 2). In some of the Pennsylvanian and Tertiary
formations, however, gastropods may be collected with the original shell
in an excellent state of preservation.
Plates 20-23 illustrate some typical Paleozoic, Mesozoic, and Cenozoic
gastropods.