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). CLASS CEPHALOPODA.—