The phylum Mollusca encompasses a large group of aquatic (water-dwelling) and terrestrial (land-dwelling) invertebrates which includes such familiar forms as the snails, clams, oysters, squids, and octopuses. Most mollusks possess a calcareous shell that serves as an exoskeleton, and these hard parts are well adapted for preservation as fossils. However, some mollusks (the slugs) have no shells, and others (the squids) have an internal calcareous shell. Because of their relative abundance and great variety, mollusks are particularly useful fossils. Moreover, the remains of certain mollusks, such as the oysters, are important rock builders. The phylum Mollusca has been divided into five classes: 1. _Amphineura_—the chitons or sea-mice; shell composed of eight valves or plates; not a common fossil. Ordovician to Recent. 2. _Scaphopoda_—the tusk-shells; shell composed of a single tusk-like valve; generally not a common fossil but locally abundant in certain Cenozoic formations. Devonian to Recent. 3. _Gastropoda_—the snails and slugs; slugs are without shells, snails have a single-valved shell which is typically coiled; common fossils in Paleozoic, Mesozoic, and Cenozoic rocks. Cambrian to Recent. 4. _Pelecypoda_—clams, mussels, oysters, scallops; shells composed of two valves, usually, but not always, of equal size; common fossils, especially in Mesozoic and Cenozoic rocks. Cambrian to Recent. 5. _Cephalopoda_—squids, octopuses, the pearly nautilus, and the ammonoids (extinct); shell of one valve, usually coiled and partitioned by septa; valuable fossils, especially in Paleozoic and Mesozoic rocks. ?Cambrian, Ordovician to Recent. [Illustration: Plate 18 PENNSYLVANIAN BRACHIOPODS] MARGINIFERA × 1 AMBOCOELIA × 1 SQUAMULARIA × 1 DERBYA × 1 MESOLOBUS × 1 CHONETES × 1 LINOPRODUCTUS × 1 PUNCTOSPIRIFER × 1 COMPOSITA SUBTILITA × 1 NEOSPIRIFER × 1 [Illustration: Plate 19 PENNSYLVANIAN BRACHIOPODS] JURESANIA × 1 SPIRIFER ROCKYMONTANUS × 1 NEOSPIRIFER CAMERATUS × 1 Of these five classes, only the Gastropoda, Pelecypoda, and Cephalopoda are discussed herein. CLASS GASTROPODA.— The typical gastropod has a spirally coiled, single-valved, unchambered shell. This shell encloses a soft body possessing a well-defined head with a pair of eyes and one or two pairs of tentacles. Most gastropods have gills and live in shallow marine waters, but some inhabit fresh water. Others are land-dwelling forms and breathe by means of lungs.