MEASUREMENT. The most approved method of stating the measurements of various kinds of shells is as follows: _symmetrical convolute univalves_, the length is from anterior to posterior; the depth from ventral to dorsal; the breadth, from side to side of the aperture. Of _symmetrical conical univalves_, length, from front to back; breadth from side to side; depth from apex to base. Of _spiral univalves_, length, from apex to anterior of the columella or axis of the shell; breadth, across from the outer lip to the opposite side. Of _non-symmetrical bivalves_, the length is from the anterior to the posterior margin; breadth, from the greatest convexity of one valve to the corresponding part of the other; depth, from the ventral to the dorsal margin. MEGADESMA. Bowd. ([Greek: Megas], _megas_, great; [Greek: desma], _desma_, ligament.) POTAMOPHILA, Sow. GALATHÆA, Lam. MEGADOMUS. Sw. A sub-genus of Unio, thus described: "Only one lateral tooth in each valve; cardinal teeth two; posterior hinge margin winged. M. gigas, _Sw._" Sw. p. 378. MEGALODON. Sow. ([Greek: Megas], _megas_, great; [Greek: odos], _odos_, tooth.) _Fam._ Cardiacea, Lam.--_Descr._ Equivalve, longitudinal, acuminated at the umbones, thick; hinge forming an incrassated septum across the cavity of the shell, with a large bifid tooth in the right valve, and one irregular and one pointed in the left; ligament long, external.--_Obs._ The general form, the thickened hinge reaching across the cavity of the valve and the terminal umbones serve to distinguish this genus from Cardita, to which, however, it is nearly allied. M. cucullatus, fig. 127. MEGALOMASTOMA. Guild. A sub-genus of Cyclostoma, thus described: "Cylindrical, resembling _Pupa_, but has a horny operculum; spire not thickened; teeth or fold on the pillar none, flavula _Sw._ En. Méth. 461. f. 6, brunnea _Guild._ (_fig. 97. g. h. 1._)" Sw. p. 336. Mr. Gray applies the name to those species which have "a groove or ridge in front of the mouth near the pillar." MEGARIMA. Rafinesque. A genus proposed to include species of TEREBRATULA, Auct. which are smooth and nearly equivalve. T. lævis, T. crassa, T. truncula. MEGASPIRA. Lea. ([Greek: Megas], _megas_, great, and spire.) M. Ruschenbergiana, (fig. 294) is a pupiform land shell remarkable for the length of its spire, which consists of no less than twenty-five close set, narrow, gradually increasing whorls. The outer lip is simple, slightly thickened; the inner lip has a tooth on the body-whorl, and two folds on the columella. Only one species of this singular shell is known. MEGATREMA. Leach. A genus composed of those species of Pyrgoma, Auct. which have a large aperture. Fig. 33. MELACANTHA. Sw. A sub-genus of Melania. Sw. p. 341. MELAFUSUS. Sw. A sub-genus of Melanopsis. Sw. p. 341.