ME(*ALOMASTOMA, 197 



MAEPESSA. Gray. A sub-genus of Clausilia, C. bidena, &c. 

 Auct. Gray's Turton, p. 212. 



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

 XAS, Auct. which are described as short, cuneiform, nea,rly closed 

 at both extremities, having several accessary pieces on the middle 

 of the back, and two marginal, lower down. The Monograph of 

 the genus Pholas, in Sowerby's Thesaurus ConchyKorum, No. 

 10. Ph. striata, sp. 29, fig. 40 to 44, and other similar species. 



MEASUEEMENT. The most usual method of stating the 

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

 cal convolute univalves, the length is from anterior to posterior ; 

 the depth from ventral to dorsal ; the breadth, from side to side 

 of the aperture, as marked on the diagram, page 18. Of sym- 

 metrical 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. See Introduction. 



MEGADESMA. Bowd. (Mtyag, megas, great ; Se<r//a,. desma, 

 ligament.) Potamophila, Sow. Galath^a, Lam. 



MEGADOMUS. Sw. A sub-gnus of Fnio, thus described: 

 " Only one lateral tooth in each valv^e ; cardinal teeth two ; 

 posterior hinge margin winged. M. gigas, Sw.'" Sw. p. 378. 



MEGALODON. Sow. (Msyae, we^as, great ; oIoq, 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 ; 

 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, PL vii. fig. 127. 



MEGALOMASTOMA. Guild. A sub-genus of Cyclostoma, thus 



