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Show 380 DR. U. L. JAMESON ON THE [Apr. 16, Section b. Teeth very rudimentary. Shell fiat as in M. margaritifera. Posterior marginal notch absent or slightly developed. Rostrum not sharply marked off from the inner surface of the valve. Longest dorso-ventral axis perpendicular to the hinge-line. Species 4. MARGARITIFERA SUGILLATA. Avicula sugillata, Reeve, 1857, no. 27; Cape Hillsborough. Type B.M. Avicula fimbriata, Reeve (nee Dunker), 1857, no. 25 ; N . W. Australia (nom. precocc). Type B.M. Avicula (Meleagrina) reeviana, Dunker, 1872, p. 45, tab. 15. fig. 1 (after Reeve), for A. fimbriata, Reeve. Avicula irradians, Reeve, 1857, sp. 35 ; Australia. Type B.M. The types of Reeve's three species are all very young shells. There are a few older examples in the Museum, acquired since the date of Reeve's monograph. I also possess a series of this shell, which I collected in Torres Straits. The complete inter-gradation between Reeve's three species makes it quite impossible to regard them as distinct. I regard M. sugillata as the Torres Straits representative of the " Sharks Bay shell" of the London markets. The latter I am describing under a distinct name, as a well-marked geographical race, even if not actually specifically distinct. The form of the adult M. sugillata is very rariable, approximating to that of the Sharks Bay sheli, but as a rule more convex, and with a shorter hinge relatively to the size of the valves. The colour is a dirty greyish yellow, with four or five brownish radial bands, which may be indistinct and imperfect, or may fuse to give the shell a uniform dark colour. The lappet-like processes of the lip are large (especially in young examples), delicate, and characteristically "crimped" so as to be more or less t/i-shaped in section, a feature easily discernible in Reeve's figures. The lappets are so thin as to be usually broken off except just at the lip ; but here they are generally crowded together in grown shells, giving the lip a curious thickened appearance, which I have elsewhere seen only in specimens of M. margaritifera grown in unsuitable, silt-laden water. The inside of the lip is dirty yellow, with cloudy brown markings, corresponding to the dark radial bands. These markings show a strong tendency to fuse and form a dark zone on the inner edge of the lip, next to the nacre. The nacre is white, with yellowish or greenish tints, but somewhat lacking in lustre. Cape Hillsborough, N . W . Australia (Reeve). Port Essington and Torres Straits (spp. in Museum). Prince of Wales Island aud Thursday Island, Torres Straits (H. Lyster Jameson). This species, although closely resembling the Sharks Bay shell, has not yet found its way into the markets. |