MARGINIFERA × 1 AMBOCOELIA × 1 SQUAMULARIA × 1 DERBYA × 1 MESOLOBUS × 1 CHONETES × 1 LINOPRODUCTUS × 1 PUNCTOSPIRIFER × 1 COMPOSITA SUBTILITA × 1 NEOSPIRIFER × 1
Plate 19
[PENNSYLVANIAN] BRACHIOPODS
JURESANIA × 1 SPIRIFER ROCKYMONTANUS × 1 NEOSPIRIFER CAMERATUS × 1
Of these five classes, only the Gastropoda, Pelecypoda, and Cephalopoda are discussed herein.
CLASS GASTROPODA.—
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 This plate effectively seals the aperture when the animal is withdrawn into its shell.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. 16]a). 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.