Plates [20]-23 illustrate some typical [Paleozoic], [Mesozoic], and [Cenozoic] gastropods.
CLASS PELECYPODA.—
The pelecypods possess a shell composed of two [calcareous] valves ([fig. 17]) which enclose the soft parts of the animal. Members of this class live exclusively in an aquatic [habitat] and are most abundant in marine environments. Most pelecypods are slow-moving bottom-dwelling forms, but some, like the oysters, are attached. Still others, for example, the scallop or Pecten, are swimmers. The Pelecypoda include such familiar saltwater forms as the clams and oysters, as well as the common fresh-water mussel. Pelecypods range from [Cambrian] to Recent in age but are more abundant in [Mesozoic] and [Cenozoic] rocks.
The living animal is aquatic, with well-developed soft parts and a muscular, commonly hatchet-shaped foot. The soft [mantle] encloses the body and secretes the shell, and in some pelecypods part of the mantle is developed into the incurrent and excurrent siphons. The incurrent siphons bring fresh water and food into the mantle cavity, and waste products are passed out through the excurrent siphons. [Respiration] is by means of gills within the mantle cavity.
The typical [pelecypod] valves are of equal size and form, but some, such as the scallops and oysters, have two valves of unequal size and shape. The valves are hinged and held together by a tough elastic ligament which runs along the [dorsal] (top) side of the shell. In addition to the ligament, most forms have teeth and sockets which are located along the [hinge-line]. The teeth in one [valve] articulate with the sockets in the opposite valve, and this arrangement gives strength to the hinge.
Fig. 16. Morphology and principal parts of [gastropod] shells. (a) Low-spired form with [umbilicus]. (b) Section of spirally coiled shell showing [columella].
a [Suture] [Whorl] Body whorl [Aperture] [Umbilicus] b Apex Spire [Columella] Body whorl Inner lip Outer lip
Most of the [pelecypod] shell is of calcium carbonate, but the outer layer, or [periostracum], of each [valve] is composed of horny material. The inner surface of the shell is lined with a [calcareous] layer of pearly or [porcelaneous] material.
Plate 20
[PENNSYLVANIAN] GASTROPODS