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Show 376 DR. H. L. JAMESON ON THE [Apr. 1°\ extreme. Posterior angle, formed by the meeting of the hinge and posterior margin of nacre, slightly obtuse, intermediate between that of M. m. typica and v&v. persica. Colour slightly darker than in var. persica. White radii very distinct. Inside of lip olive-green or brown. Nacre as in tax.persica but less silvery. Size as iu typica. Hab. Red Sea. A specimen in the Museum from Aden, presented by Major Yerbury. [Mother-of-Pearl dealers recognize three classes of Egyptian shells, differing slightly in colour and lustre, viz., Jiddah (darkest), Massowah (medium), and Aden (lightest). The last most closely resembles var. persica.~\ Var. e. M. margaritifera cumingi. Avicula cumingii, Reeve, 1857, sp. 6 (Lord Hood Island). Type B.M. The Black-edged shell of Eastern Polynesia, known in the trade as ' Tahiti," " Gambier," " Auckland'," Ac. shell. Form as in M. m. typica, but attaining greater dimensions ; at times as much as 24-30 cm. in diameter, and often weighing six to eight pounds per pair of valves. Colour deep glossy black, with, at most, very obscure traces of the radial rows of spots. The latter are often quite suppressed. Nacre steely in lustre, with a very broad margin of dark metallic green, this border being wider and darker thau in M. m. typica. The " Tahiti," " Gambier," and "Auckland" shells of the trade are distinguished by slight differences in form, but in colour they all conform to M. cumingii of Reeve. Geographical Distribution. Tahiti; Gambier Arch. ; Lord Hood Isl.; Penrhyn Group, and Eastern Polynesia generally. I am not aware where the " Auckland " shells of the London markets are fished. Their name is probably due to their being shipped per Auckland from some Polynesian locality. Var. f. M. margaritifera mazatlanica. Margaritiphora mazatlanica, Hanley, 1855, p. 388, pi. 24. fig. 40 (Mazatlan, California). Avicula barbata, Reeve, 1857, sp. 9 (Panama). Type B.M. " Panama shell" of the trade. This shell at first sight suggests a dwarfed and very convex example of M. maxima rather thau a geographical race of M. margaritifera. It seems, however, to iotergrade with the latter. The distinctive characters of this variety are its great convexity and its aberrant shape and colour. The posterior angle is acute, or, more rarely, a right angle ; and the posterior margin of the nacre slopes forward from the hinge, much as in M. maxima, so that a perpendicular to the hinge, at its posterior end, would fall entirely posterior to the border of |