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Show 372 DR. H. L. JAMESON ON T H E [Apr. 16, 4. On the Identity and Distribution of the Mother-of-Pearl Oysters ; with a Revision of the Subgenus Margaritifera. By H. LYSTER JAMESON, B.A., Ph.D. [Received April 2, 1901.] (Text-figures 92-95.) Acting upon a suggestion made to me by Professor E. Bay Lankester,! have recently been engaged in rearranging the collection of Margaritiferos in the British Museum of Natural History, and iu revising and extending the series illustrating the commercial forms. The nomenclature of these forms has hitherto been in a chaotic state, and it has occurred to me that this revision of the genus may be worth publication if only to prevent further confusion of the common commercial forms by zoological and economic writers. The subgenus Margaritifera (P. Browne = Meleagrina Lamarck) is one of the three groups into which the genus Pteria (Scopjli = Avictda Olivi) has been divided. It is distinguished from Pterin s. s. mainly by the small size or complete absence of the posterior auricle or hinge-continuation, and by the fact that the longest dorso-ventral axis of the valves is approximately at right angles to the hinge-line. Iu Pteria s. s. and Electroma Stoliczka (type E. smaragdina, Bceve), on the other hand, the shell is oblique, i. e. the long axis of the valves is directed obliquely backwards. Electroma is distinguished from Pteria s. s. by the shortness of its hinge and the absence of the posterior auricle. Such species as Margaritifera vulgaris and M. pauasesn1, together with certain Pterin and Eleetromce, tend in many ways to bridge over the gaps between these three subgenera. Grenus PTERIA Scopoli, 1777, p. 397 : type Mytilus hirundo Linu. =Avicula Olivi, 1792, p. 125. Subgenus MARGARITIFERA P. Browne, 1756, p. 412. (JNon binom.: type M. radiata Leach : Jamaica.) = Margaritifera Humphrey, 1797 (ex parte), p. 44 : type M.fim-briata (not descr.) ( ? = A7. radiata Leach); W . Indies. Apud Da Costa, 1776. = Margaritophora Megerle von Muhlfeld, L811, p. 66: type M. communis =M, margaritifera Linmeus. = Meleagrina Lamarck, 1812, p. 104; 1819, p. 150: type M. margaritifera Linmeus. = Margarita Leach, 1814, vol. i. p. L07 : type M. sinensis Leach = M. margaritifera Linnaeus ; China. = Perlamater Schumacher, 1817, p. 107 : type P. vulgaris Schumacher. The species of Margaritifera are difficult to separate from one |