DISTORSIO × 1 MESALIA × 1 FUSUS × 1 COCHLESPIROPSIS × 1 TURRITELLA × 1 LATIRUS × 1 CONUS × 1 VERTAGUS × 1 PSEUDOLIVA × 1
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. 17]a).
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. 17]b). In addition to hinge teeth, certain [species] have cardinal teeth which are located below and in front of the hinge teeth.