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