1881.] L.A.KES TANGANYIKA AND NYASSA, ETC. 297 



locality vary considerably ; however, not one of them takes the 

 form of U. ceffijptiacus 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 miiform 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. 33-336.) 



Unio hurtoni. Woodward, Proc. Zool. Soc. 1859, p. 349, pi. 47. 

 fig. 1. 



Hah. 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 Mu- 

 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 epi- 

 dermis 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 dif- 

 ferent 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 sculp- 

 ture is intermediate between that found on the more rugose variety 



Proc. Zool. Soc— 1881, No. XX. 20 



