PERIOSTRACUM. A name used by Mr. Gray to signify the substance which covers the outer surface of many shells, called the _Epidermis_ by most conchological writers. "Drap Marin" is the name given to this substance by French Naturalists. PERIPLOMA. Schum. _Fam._ Myariæ. A genus thus described: "Shell very thin with the left valve more ventricose than the right; hinge toothless, ligament double, the external portion thin, the internal part thick, placed upon prominent, sometimes spoon-shaped hinge laminæ, and supported by a transverse bone; muscular impressions two, distant, palleal impression sinuated posteriorly." _Ex._ P. inæquivalvis. fig. 72. _Genus_, Osteodesma, Deshayes. PERISTOMATA. Lam. A family belonging to the first section of the order Trachelipoda, containing the following genera:-- 1. AMPULLARIA. Globose or discoidal; operculum concentric; including _Pachystoma_, _Lanistes_, _Ceratodes_. Fig. 318 to 320. 2. PALUDINA. Oval; operculum concentric. Fig. 321. 3. VALVATA. Globose; operculum spiral. Fig. 322. PERISTOME. The edge of the aperture, including the inner and outer lips. PERITREME. A term used to express the whole circumference of the aperture of a spiral shell. It is said to be notched or entire, simple, reflected, round or oval, &c. PERLAMATER. Schum. (_Mother of Pearl._) MELEAGRINA Margaritifera, Lam. The pearl oyster. PERNA. Auct. ("Pernæ concharum generis," Plin.) _Fam._ Malleacea, Lam.--_Descr._ Sub-equivalve, irregular, compressed, foliaceous; hinge straight, linear, composed of a series of transverse, parallel grooves, containing the cartilage and intermediate spaces bearing the ligament; anterior margin with a sinus for the passage of a byssus; posterior ventral margin oblique, attenuated. _Obs._ This genus is known from Crenatula by the straightness, number and regularity of the grooves in the hinge and the sinus, for the passage of the byssus. Fig. 166, P. Ephippium. Mostly tropical. PERSICULA. Schum. A genus formed of MARGINELLA _Persicula_, Auct. and other species having the spire concealed. Fig. 438. PERSONA. Montf. (_Mask_). A genus composed of TRITON _Anus_, Auct. and similar species. Fig. 401. PETRICOLA. Lam. (_Petrus_, a stone; _cola_, an inhabitant.) _Fam._ Lithophagidæ, Lam.--_Descr._ Equivalve, inequilateral, transversely ovate or oblong, rather irregular, anterior side rounded; posterior side more or less attenuated, slightly gaping; hinge with two cardinal teeth in each valve; muscular impressions two in each valve; palleal impression entire; ligament external.--_Obs._ The Petricolæ are found in holes made by the animals in rocks, madrepores, &c. They may be known from Saxicava by the regularity of their form and the teeth on the hinge. Fig. 91, 92.