Hinge-line—in brachiopods, the edge of the shell where the two valves articulate; in pelecypods, the dorsal margin of the valve which is in continual contact with the opposite valve. Igneous rock—rocks which have solidified from lava or molten rock called magma. Index fossil—see Fossil. Inequivalved—opposite valves unlike in shape or size, or both. Jurassic—second oldest period of the Mesozoic; follows the Triassic, precedes the Cretaceous. Keel—a strong continuous ridge along the ventral side of ammonites. Larva—the young form of some animals before they assume the mature shape. Lateral—side or to the side. Lithology—the study and description of rocks based on the megascopic (with the naked eye) examination of samples. Used also to refer to the texture and composition of any given rock sample. Living chamber—in mollusks, that part of the shell which is occupied by the living animal. Lobe—in cephalopods, the backward flexure of the suture or septum. Longitudinal—in a direction parallel with the length. Lophophore—in brachiopods, a tentacle-bearing appendage attached to the anterior surface of the mantle cavity. Mantle—in mollusks and brachiopods, a layer of tissue containing cells that secrete the shell. Meso-—a prefix signifying middle. Mesozoic—that era of geologic time that precedes the Cenozoic and follows the Paleozoic. Miocene—fourth oldest epoch of the Tertiary period; follows the Oligocene, precedes the Pliocene. Mississippian—fifth oldest period of the Paleozoic: follows the Devonian, precedes the Pennsylvanian. Multicellular—composed of more than one cell. Nacreous—pearly. Node—a knob. -oid—a suffix meaning “in the form of.” Oligocene—the third oldest epoch of the Tertiary period: precedes the Miocene, follows the Eocene. Operculum—the lid or covering of the aperture of certain shells. Oral—referring to the mouth or aperture. Orbitoidids—foraminifers with large typically disk-shaped tests. Ordovician—second oldest period of the Paleozoic era; follows the Cambrian, precedes the Silurian. Ossicle—loosely used as a small plate. Paleocene—oldest epoch of the Tertiary period; precedes the Eocene. Paleozoic—that era of geologic time that follows Precambrian time and precedes the Mesozoic era. Pedicle opening (pedicle foramen)—see Foramen. Pelecypod—a bivalved aquatic invertebrate; member of class Pelecypoda, phylum Mollusca. Pennsylvanian—the sixth oldest period of the Paleozoic era; follows the Mississippian, precedes the Permian. Period—a division of geologic time (Pl. 1). Periostracum—the horny outer covering or epidermis on shells. Permian—seventh and last period of the Paleozoic. Permineralization—that process by which mineral matter has been added to the original shell material by precipitation in the interstices rather than replacing the original shell material. Phosphatic—containing or pertaining to phosphate minerals.