The Ethiopian region may be divided into three sub-regions: (1) the Central African; (2) the South African; (3) the Malagasy.
(1) The Central African Sub-region is bounded on the north by the Great Desert, on the east and west by the ocean, and on the south by a line roughly drawn between the mouth of the Orange River and Delagoa Bay; it also includes S. Arabia. No natural features exist which tend to break up this vast district into areas of independent zoological development. The absence of long and lofty mountain ranges, the enormous size of the great river basins, and the general uniformity of climate, equalise the conditions of life throughout. It will be convenient to break the sub-region up into provinces, but in most cases no precise line of demarcation can be laid down.
(a) The Senegambian Province may be regarded as extending from the mouth of the Senegal River to Cape Palmas. Only 8 genera of land Mollusca are known, including 4 Limicolaria and 3 Thapsia, with 1 small Cyclophorus. Fresh-water genera are abundant, and include most of the characteristic Ethiopian forms.
(b) The West African Province extends from Cape Palmas to the mouth of the Congo, and is rich in Mollusca. The great Achatina, largest of land snails, whose shell sometimes attains a length of 6½ in., Limicolaria, Perideris, and Pseudachatina are the characteristic forms. The Agnatha are represented by Ennea, Streptaxis, and Streptostele. Rachis and Pachnodus, sub-genera of Buliminus, occur also on the east coast. A special feature is the development of several peculiar slug-like genera, e.g. Oopelta, perhaps a form of Arion; Estria, a slug with an external shell, akin to Parmacella; and Aspidelus, a form intermediate between Helicarion and Limax. Claviger, a handsome group akin to Cerithium, is peculiar to the estuaries of West African rivers.
About sixteen species are known from the Cameroons District, but no peculiar genera occur. The French Congo District has not yet been well explored. Tomostele, a genus allied to Streptostele, is peculiar, and Pseudachatina attains its maximum.
Fig. 217.—Columna flammea Müll., Princes I.
St. Thomas and Princes Is., in the Gulf of Guinea, are well known. Princes I. has 22 species, 14 peculiar, and 2 common to St. Thomas only, one of the latter being the great sinistral Achatina bicarinata Chem. The remarkable genus Columna (Fig. [217]) is peculiar, and Streptostele (4 sp.) attains its maximum. Peculiar to St. Thomas are Pyrgina, a turreted form of Stenogyra; Thyrophorella, a sinistral form of Zonites; and Atopocochlis, a large bulimoid shell, whose true relationships are not yet known. Homorus, a group of Achatina with an elongated spire, occurring also in the Angola District and on the east coast, has 4 species. No fresh-water species have as yet been discovered in either of the islands.
The Angola and Benguela District, extending from the Congo to the Cunene R., probably belongs to the West African Sub-region, but until its fauna is better known it is advisable to consider it apart. Achatina continues abundant, but the other characteristic West African forms (Pseudachatina, Streptostele, Perideris) diminish or are absent altogether. No Helix and only 1 Cyclophorus occur.
Ovampo, Damara, and Great Namaqualand, lying between the Cumene and Orange rivers, seem to form a transition district between the West and South African faunas. Helix reappears, while the characteristic West African genera are almost entirely wanting.