332 THE GREAT LAKES chap. 



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 com- 

 pactness 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 rep- 

 resents 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 Hydrohia occur. 



Of the genera concerned, Paramelania and Nassopsis are 

 forms of Melaniidae ; Tiphohia (Fig. 219), which is allied to 

 Paludomus, is a compact shell with angulated spinose whorls ; 

 Lacunopsis, Ponsonhya^ Limnotrochus, and Tanganyieia are prob- 

 ably forms of Lithoglyphus^ some, as their names denote, being 

 of decidedly marine facies ; Syrnolopsis and Turhonilla (?) look 

 like Pyramidellidae, Horea and Reymondia like Rissoina ; Bour- 

 guignatia 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. 



(e) The Afro-Arabian Province includes Abyssinia, with S. 

 Arabia, the African shores of the Gulf of Aden, and Socotra. 

 The province contains a singular mixture of types. The high 

 ground of Abyssinia stands like a lofty European island in the 

 midst of a tropical plain, with Palaearctic genera flourishing like 

 hardy northern plants on a mountain in low latitudes. Helix, 

 Vitrina, and Pupa abound, with a few Qlausilia and even a 

 Limax. On the lower levels occur Limicolaria (3 sp.), Suhulina 

 (7 ^^.),Helicarion, and Homorus, but land operculates are entirely 

 wanting. Characteristic of the province as a whole are various 

 forms of Bidiminus, which in Socotra are represented by two 

 peculiar sub-genera, Achatinelloides and Passamaiella. In S. 



