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.
[Illustration: 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.
[Illustration: Plate 20
PENNSYLVANIAN GASTROPODS]
STRAPAROLUS × 1
AMPHISCAPHA × 1
WORTHENIA × 1
TREPOSPIRA × 1
BELLEROPHON × 1
EUOMPHALUS × 1
[Illustration: Plate 21]
[Illustration: PENNSYLVANIAN GASTROPODS]
EUPHEMITES × 1
STROBEUS × 1
PLATYCERAS × 1
[Illustration: CRETACEOUS GASTROPODS
INTERNAL MOLDS]
GYRODES × 1
LUNATIA × 1
TURRITELLA × 1
CERITHIUM × 1
TYLOSTOMA × 1
NERINEA × 1
[Illustration: Plate 22
TERTIARY GASTROPODS]
DISTORSIO × 1
MESALIA × 1
FUSUS × 1
COCHLESPIROPSIS × 1
TURRITELLA × 1
LATIRUS × 1
CONUS × 1
VERTAGUS × 1
PSEUDOLIVA × 1
[Illustration: Plate 23
TERTIARY GASTROPODS]
ANCILLA × 1
ARCHETECTONICA × 1
TUBA × 1
CALYPTRAPHORUS × 1
SYCOSTOMA × 1
SURCULA × 1
VOLUTOLITHES × 1
NEVERITA × 1
LEVIFUSUS × 1
The outline of the shell may vary greatly, but most pelecypods are
typically clam-like. However, certain forms are round, others are long
and narrow, and some have wing-like structures. Most pelecypods have a
beak which represents the oldest part of the shell. The _beak_ is
commonly located on the _anterior_ (front) end of the shell, and the end
of the shell opposite this is designated _posterior_ (the rear). The
hinge and ligament are located dorsally (along the top), and the lower
margin of the shell where the valves open is called _ventral_ (fig.
17a).
[Illustration: Fig. 17. Morphology and principal parts of a typical
pelecypod shell. (a) Exterior view. (b) Interior view.]
a
Dorsal
Beak
Anterior
Posterior
Concentric growth rings
Ventral
b
Hinge teeth
Cardinal teeth
Anterior muscle scar
Posterior muscle scar
Mantle line
The inner surface of the shell has certain markings which, along with
the shell form and dentition (the nature and arrangement of the teeth
and sockets), are important in classification. Muscle scars are present
on the inside of most valves; the _anterior muscle scars_ are located
near the front of the shell, and the _posterior muscle scars_ are
situated near the rear of the shell. These scars mark the place of
attachment of muscles which were used to close the shell and aid in
locomotion. Along the ventral margin of some shells there is a line or
scar which extends from the anterior muscle scar to the posterior muscle
scar. This is known as the _mantle line_ or _pallial line_ and marks the
place of attachment of the _mantle_—a soft membranous layer that
enclosed the body of the animal. In some pelecypods the dorsal margin of
one valve bears a series of _hinge teeth_ which articulate with a
similar set of sockets on the other valve (fig. 17b). In addition to
hinge teeth, certain species have _cardinal teeth_ which are located
below and in front of the hinge teeth.
The exterior of most shells is marked by a series of _concentric growth
lines_ (fig. 17a) which mark points of periodic addition of shell
material. The external surface of many shells is also marked by various
types of ornamentation, such as ribs, nodes, spines, and grooves.
Fossil collectors commonly find only one valve of the pelecypod shell.
This is because the shell normally opens when the animal dies, and the
valves may easily become separated. Fossil pelecypods are also commonly
preserved as external and internal molds, and these are found in
fossiliferous strata of almost all ages. Some pelecypods of
Pennsylvanian, Mesozoic, and Cenozoic age are found with original shell
material that appears to have undergone very little change. Fossil
pelecypods are abundant and varied in Texas and are found in most of the
fossiliferous formations of the Pennsylvanian, Cretaceous, and Tertiary
systems (Pls. 24-31).