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Show 1881.] LAKES TANGANYIKA AND NYASSA, ETC. 297 locality vary considerably; however, not one of them takes the form of U. a?gyptiacus of Ferussac, which Jickeli unites with this species. All have the more rhomboidal form and less central umbones of U. niloticus; and the epidermis varies from dark olive-brown to yellowish olive or uniform brown. Some of the specimens are indistinctly rayed ; and others have a few raised epidermal ridges radiating from the umbones towards the anterior end of the ventral margin. Without exception the valves in front of the umbones are corrugated; but posteriorly there is but rarely any trace of such ornamentation. 50. UNIO BURTONI, Woodward. (Plate XXXIV. figs. 33-335.) Unio burtoni, Woodward, Proc. Zool. Soc. 1859, p. 349, pi. 47. fig. 1. Hab. Tanganyika (Hore and Thomson). The collection brought home by Mr. Thomson contains a series of specimens which I believe are varieties of this species. Not one of them corresponds with the typical form, of which the M u seum possesses two examples-one from the Cumingian collection, the other, the type, presented by Capt. Speke. Both these shells are in a considerably worn condition, scarcely a vestige of the epidermis remaining. Their form, too, is more transverse, flatter; and the umbones are less prominent. Among the present series there are two varieties, one much more strongly sculptured than the other. The subgranular character of this ornamentation is somewhat different from that on the typical shells; but some allowance must be made for this, owing to their worn condition. Their outline, too, is no doubt modified to a certain extent from the same cause. The colour of the interior varies from white to uniform rich brownish purple. Both varieties vary in this respect; and intermediate grades of coloration are found in each series. The exterior of the valves also presents difference of colour, in some specimens being of a yellowish tint indistinctly radiated with fine green lines; others are of the same general colour, but lack the green rays; and, again, certain specimens exhibit more of an olivaceous exterior more or less varied with green, the latter consisting of indistinct fine radiating lines. As a whole, the form of Mr. Thomson's specimens is rather more convex and deeper from the umbo to the ventral margin than that of the two typical specimens; still, as I have previously stated, this in the latter case may be the result of their worn state. Other specimens, collected by Mr. Hore near Ujiji and recently forwarded to the British Museum by Dr. John Kirk, approach the typical form more closely. The outline varies very much, judging from this series, the position of the umbones also being in some much more forward than in others. The freshest of these, having the epidermis to some extent preserved, shows that the species is ornamented with fine green radiating lines. The amount of sculpture is intermediate between that found on the more rugose variety PROC. ZOOL. Soc-1881, No. XX. 20 |