186 MEGALOMASTOMA. 



MARTESIA. Leach. A genus composed of those species of Pho- 

 las, Auct. which are described as short, cuneiform, nearly 

 closed at both extremities, having several accessary pieces on the 

 middle of the back, and two marginal, lower down. 



MEASUREMENT. The most approved method of stating the 

 measurements of various kinds of shells is as follows : symme- 

 trical 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. (Meyag, megas, great ; Italia, desma, 

 ligament.) Potamophila, Sow. Galath^ea, 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. 3/8. 



MEGALODON. Sow. (Meyag, megas, great ; oSog, odos, tooth.) 

 Fam. Cardiacea, Lam. — Descr. Equivalve, longitudinal, acumi- 

 nated 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 ; liga- 

 ment 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. Meth. 4G1. f. 6, brunnea Guild. (Jig. 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." 



