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.