1. AVICULA. Hinge linear, simple, including _Meleagrina_. Fig. 163, 164. 2. PERNA. Hinge with linear grooves, including _Pulvinites_. Fig. 166, 170. 3. GERVILLIA. Shaped like Modiola, with irregular grooves. Fig. 162. 4. CRENATULA. Hinge with a series of pits. Fig. 168. 5. CATILLUS. Like Perna, but more regular and convex. Fig. 167. 6. MALLEUS. A triangular disc on the hinge, and two auricles. Fig. 165. MALLEUS. Auct. (_Malleus_, a hammer.) _Fam._ Malleacea, Lam. Margaritacea, Bl.--_Descr._ Equivalve, inequilateral, foliaceous, trilobate, undulated, irregular, attached by a byssus passing through a sinus in one valve; hinge rectilinear, lengthened by two auricles; with a small disc under the umbones, containing the ligament, and a groove containing the cartilage; muscular impressions one in each valve, large, uniform, and one or two others extremely minute.--_Obs._ Malleus Vulgaris, the type of this genus, is a most singular shell, commonly called the "Hammer Oyster," from the peculiarity of its shape. It belongs to the Linnæan genus Ostrea, from which it differs in being attached by a byssus. Fig. 165, M. Vulgaris. Tropical. MAMILLARIA. Sw. A sub-genus of NATICA, corresponding with Polinices of Montfort, having the spire small and the umbilicus filled. _Ex._ Natica Mamilla, Auct. fig. 327. MAMMILLATED. (_Mammula_, a little teat.) A term applied to the apex of a shell when it is rounded like a teat. _Ex._ Voluta Vespertilio, fig. 433. MARGARITA. Leach. (_Margarita_, a pearl.) A genus of small shells resembling the genus Trochus, from which it differs in having an operculum consisting of few whorls. M. tæniata, fig. 362. Mr. G. B. Sowerby, sen. has enumerated 15 species in a list accompanying the figures published by the author of this manual in Nos. 132 to 134 of his Conchological Illustrations. MARGARITACEA. Bl. The third family of Lamellibranchiata, Bl. The shells belonging to it are described as irregular, inequivalve, inequilateral, black or horny without, pearly within; hinge auriculated, scarcely developed, and without teeth. The ligament is variable and there is a large sub-central muscular impression. This family contains the genera Vulsella, Malleus, Pinna, Crenatula, Inoceramus, Catillus, Pulvinites, Gervillia and Avicula. MARGARITACEOUS. (_Margarita_, a pearl.) Pearly. MARGARITANA. Schum. A sub-genus of Uniones, composed of species having "one cardinal tooth." ALASMODON, Say. MYA Margaritifera, Linn. MARGARITIFEROUS. (_Margarita_, pearl; _fero_, to bear.) Pearl-bearing. Applied to shells which form pearls; as Meleagrina Margaritifera, or Pearl-bearing Oyster. MARGINAL. Near the margin or edge.