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Show 1901.] MOTHER-OF-PEARL OYSTERS. 373 another by hard-and-fast lines, owing to the absence of well-marked diagnostic characters, and to the extraordinary amount of geographical and casual variation. Probably the anatomical characters will, when known, be found to be the best guides to the limits and affinities of some of the species. It is possible to divide the known species aud forms into two groups-the first of which has no traces of hinge-teeth, the second possessing a pair of minute tubercles anterior to the ligament, and a small lamina in each valve representing a lateral tooth. The latter group may be split up into three sections, according to the form of the shell. DIVISION 1. Hinge without teeth. Species 1. MARGARITIFERA MARGARITIFERA. Type in Mus. Linn. Soc. Lond. Mytilus margaritiferus, Linnaeus, 1760. Margarita sinensis, Leach, 1814, vol. i. p. 180, pi. 48; China. Margaritiphora mazatlanica, Hanley, 1855, p. 388, pi. 24. fig. 40; Mazatlan, California (var. dist.). (Not Avicula fimhriata Dunker, 1852; for M. mazatlanica Hanley, Carpenter, 1857, p. 550; Mazatlan.) Avicula, cumingii, Reeve, 1857, sp. 6 ; Lord Hood Island (var. dist.). Avicula barhata, Reeve, 1857, sp. 9 ; Panama = var. mazatlanica (Hanley). This species is distinguished from M. maxima, the only other known member of this division, by its greater convexity and the shortness of its binge, and by its colour and markings. In typical examples the hinge-margin measures but little more than half the length of the nacreous surface of the valve, from the anterior to the posterior margin. The " rostrum " or anterior angle of the hinge, dorsal to the byssal notch, is more distinctly marked off from the shell than in M. maxima, and the posterior end of the hinge usually meets the posterior margin so as to form an obtuse angle (except in var. persica and var. mazatlanica). There is no sinus in the posterior margin. The anterior margin, ventral to the byssal notch, projects further forward than in M. maxima, so that a line perpendicular to the hinge at its anterior end would cut off a considerable area of the nacre anteriorly (1 total antero-post. measurement). The lappet-like processes of the lip are more numerous, more crowded together, narrower, and relatively longer than in M. maxima. The greatest antero-posterior measurement of the nacre, parallel to the hinge, is about halfway between the hinge and the ventral margin, giving the shell a fairly circular outline. The colour and markings of the exterior, though extremely PROC ZOOL. Soc-1901, VOL. I. No. XXV. 25 |