MOTHER OF PEARL. This beautiful substance, which is so much resorted to for ornamental purposes, constitutes the thickened coating of the internal surface of the shell named by scientific collectors, Meleagrina Margaritifera, commonly called the Pearl Oyster, a young specimen of which is figured (164) in our plates. The reason why this substance is called mother-of-pearl is that the true pearls are produced from its surface. They arise principally from accident or disease, and are sometimes artificially produced by pricking through the outside of the shell while the animal is living. The animal is allowed to live until it has formed a pearl over the wounded part. MOULINSIA. Grateloup. PUPINA, Vignard. A genus of small land shells with enamelled surface and spiral operculum. See PUPINA. MOURETIA. Gray. "_Gadin_," Adanson. A genus of patelliform shells, described as differing from SIPHONARIA (the original Mouretia of Adanson) in the situation of the siphon, which in Mouretia is close to the place where the muscular impression is interrupted to leave a space for the head; while in Siphonaria it is nearly half way between the anterior and posterior ends of the shell. MOUTH. The aperture or opening of univalve shells. MULINIA. Gray. A genus composed of species of MACTRA, Auct. described as having the ligament (properly so called) internal, and lateral teeth simple. _Ex._ fig. 82. M. bicolor; Mactra, Auct. MULLERIA. Fer. _Fam._ Ostracea, Lam.--_Descr._ Irregular, subquadrate, inequivalve, inequilateral, foliaceous, attached, pearly within, green, horny without; hinge irregular, with a partly external ligament, passing to the interior, through a sort of sinus.--_Obs._ This remarkable shell resembles Etheria in general form and appearance, but is distinguished by having only one muscular impression. It is so rare that, although not very beautiful, a specimen has been known to produce £20. at a sale. Fig. 192. MULTILOCULAR. Many chambered. MULTISPIRAL. (_Multus_, many, _spira_, spire.) A term applied to a shell when the spire consists of numerous whorls; or to an operculum of numerous volutions. MULTIVALVE. (_Multus_, many; _valva_, valve.) Consisting of numerous valves. There are three kinds of multivalve shells: 1st. Those in which the valves are arranged in pairs, and produce a flattened figure, as Pedunculated Cirripedes, fig. 34 to 43; 2nd. Those in which they are arranged circularly, as Sessile Cirripedes, the valves of which are of two kinds; the _opercular_, consisting of several valves, which close the aperture, and the _parietal_, consisting of those which surround the body of the animal in a circular form, fig. 14 to 33. 3rd. Those in which they are arranged in a straight line, as Chiton, fig. 227.