(c) The East African Province extends from about Delagoa Bay to the Abyssinian shores of the Red Sea. In general out-line the province consists of a flat marshy district, extending inland for many miles from the sea; this is succeeded by rising ground, which eventually becomes a high table-land, often desolate and arid, whose line of slope lies parallel to the trend of the coast. The Mollusca are little known, and have only been studied in isolated districts, usually from the discoveries of exploring expeditions.
The Mozambique District, from Delagoa Bay to Cape Delgado, includes no genus which does not occur on the west coast, except Cyclostoma (2 sp.). Trochonanina (4 sp.), Urocyclus, a characteristic African slug (2 sp.), Rachis (6 sp.), Pachnodus (2 sp.), and Achatina (5 sp.), are the principal groups.
Fig. 218.—Urocyclus comorensis Fisch., Comoro Is.: G, Generative orifice; M, mucous gland; O, orifice leading to internal shell; P, pulmonary orifice; T, tentacles. (After Fischer.)
The Zanzibar District, from Cape Delgado to the Somali country, has the same general features. Meladomus, a large sinistral Ampullaria, is characteristic, while Cyclostoma (5 sp.) becomes more abundant. Helix is still absent, but the carnivorous forms (Streptaxis 2 sp., Ennea 7 sp.) are rather numerous.
The Somali District is characterised by operculate groups of the Otopoma type (Georgia, Rochebrunia, Revoilia) whose generic value is rather doubtful. Petraeus, in an Arabian type, supplants Rachis and Pachnodus. Achatina is nearly wanting, but Limicolaria has 9 species. A few Helix, said to be of the Pisana group, occur.
The District between the Great Lakes and the coast region is fairly well known through recent explorations, especially those associated with Emin Pasha. Streptaxis (6 sp.) and Ennea (24 sp.) are numerous, Helix is wanting, and the Naninidae are represented by Trochonanina (7 sp.), and other forms at present grouped under Nanina or Hyalinia. On the high ground Buliminus, Cerastus, and Hapalus replace, to some extent, the Achatina and Limicolaria of the marshy plains. Land operculates (Cyclophorus 1, Cyclostoma 8) are more numerous; among fresh-water genera we have Lanistes (5 sp.), Cleopatra (3 sp.), Meladomus (1 sp.), and Leroya, a sinistral form with the facies of a Littorina. The characteristic African bivalves (Mutela, Spatha, etc.) are few in number.
(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.
(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 Clausilia and even a Limax. On the lower levels occur Limicolaria (3 sp.), Subulina (7 sp.), Helicarion, and Homorus, but land operculates are entirely wanting. Characteristic of the province as a whole are various forms of Buliminus, which in Socotra are represented by two peculiar sub-genera, Achatinelloides and Passamaiella. In S. Arabia the mixture of types produces curious results: the Helix, Clausilia, and Vitrina being Palaearctic, the Limicolaria and all the operculates Ethiopian, while the single Trochomorpha is Indian. Indian influence, indeed, comes out unmistakably throughout the province. Thus in Socotra there are two Cyclotopsis, in Abyssinia two Africarion (closely related to the Indian Girasia), two Microcystis, and a Glessula, and in the Scioa district there is a Sitala. The fresh-water Mollusca of Socotra are Indian forms.