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Show 1881.] LAKES TANGANYIKA AND NYASSA, ETC. 277 brackish. The flavour is unpleasant ; and when other water is procurable, that of the lake is not drunk by the natives. Two of these most remarkable shells (Limnotrochus thomsoni and L. hirki) possess all the general outward aspect of the marine genera Trochus and Echinella ; and the third has a wonderful resemblance to Syrnola in the Pyramidellidae. Moreover Melania nassa (a very variable form) and M. horei (on the contrary, with very constant characters) have much more the look of marine than lacustrine species ; and it is very probable that when their animals are known they will exhibit some anatomical differences which will necessitate their removal from the Melaniidae. In describing the remarkable genus Tiphobia in m y former paper I was unable to give any account of the operculum. Fortunately one of the specimens brought home by Mr. Thomson contains that appendage; and its structure shows that the species is Melanoid, as was originally surmised. The same defect in the description of the Paludina-like Neothauma is now supplied; for several of the specimens contain opercula, which prove to be similar to that of Paludina. Altogether thirty species are now known to inhabit Tanganyika. Of these seventeen are apparently peculiar to it, nine having been recorded from other localities, chiefly more northward, in Nilotic regions. Two of these {Limna?a natalensis and Melania tuberculata) also occur in Lake Nyassa; and certain shells which appear to be varieties of Cor-bicula radiata and Unio nyassaensis also inhabit both lakes. Only two additional species are now included in the Nyassa fauna-one a new Ampullaria (a genus not previously recorded from that lake), the other the well-known Lanistes purpureus-thus raising the total number of its known species to twenty-seven. Of the land-shells hereafter described I would call special attention to the Bulimus notabilis, quite unlike any other African form, and to Streptaxis gigas and S. craveni, the former being the giant of the genus. 1. CYCLOPHORUS WAHLBERGI, Benson. Cyclophorus wahlbergi, Pfeiffer, Con.-Cab. pi. 50. f. 17-19 ; Reeve, Con. Ic. f. 81. Hab. Between Lake Nyassa and the east coast {Thomson). This well-known South-African form has not, I believe, been recorded from so northern a locality ; but two other species which are very similar have been described :-one, C. magilensis, Craven, from Magila, which I think may prove to be only the young state of this species; and the other C. hildebrandti, Martens, from Ukamba, which although in general aspect very like, still differs in its greater size and fewer whorls. In Reeve's figure the aperture is represented unusually large. 2. CYCLOSTOMA INSULARE, Pfeiffer, var. (Plate XXXII. figs 1, 1 a.) Cyclostoma insulare, Pfeiffer, Proc. Zool. Soc. 1852, p. 64; Con-chyl.- Cab. p. 351.no. 368, pi. 45. figs. 5, 6 ; Reeve, Con. Ic. fig. 41. |