(_d_) _Province of the Great Lakes._--The Mollusca of the four great lakes of Eastern Central Africa--Lakes Albert Nyanza (Luta Nzige, 2720 ft.), Victoria Nyanza (Oukéréwé, 3700 ft.), Nyassa (1520 ft.), and Tanganyika (2800 ft.)--are well known, and supply an interesting problem in distribution. Those of the three first mentioned lakes differ in no way from the rest of tropical Africa, but the Mollusca of Tanganyika include, in addition to the ordinary African element, a number of peculiar operculate genera, belonging principally to the Melaniidae and Hydrobiidae. Several of these possess a solidity of form and compactness of structure which is unusual in fresh-water genera, and has led to the belief, among some authorities, that they are the direct descendants of marine species, and that Tanganyika represents an ancient marine area. This view appears untenable. The Victoria Nyanza and Nyassa are part of the same system as Tanganyika, and it is not easy to see how, if Tanganyika were once an arm of the sea, they were not equally so, especially as they are several hundred miles nearer the Indian Ocean as at present defined. Nor, as will be seen from the figures given above, is there anything in the altitudes which would make us expect anything exceptional in Tanganyika. The similar case of L. Baikal must be compared (p. 290), where again a number of specialised forms of _Hydrobia_ occur. Of the genera concerned, _Paramelania_ and _Nassopsis_ are forms of Melaniidae; _Tiphobia_ (Fig. 219), which is allied to _Paludomus_, is a compact shell with angulated spinose whorls; _Lacunopsis_, _Ponsonbya_, _Limnotrochus_, and _Tanganyicia_ are probably forms of _Lithoglyphus_, some, as their names denote, being of decidedly marine facies; _Syrnolopsis_ and _Turbonilla_ (?) look like Pyramidellidae, _Horea_ and _Reymondia_ like _Rissoina_; _Bourguignatia_ appears to belong to _Vivipara_, with which has now been merged the genus _Neothauma_. Recently discovered forms from the adjacent L. Mweru are evidently of kindred origin.