CHAP. XI CENTRAL AFRICAN SUB-REGION 329 



number of other genera abundant along the Mediterranean, are 

 either altogether absent, or are very scantily represented. Land 

 operculates,so characteristic of other tropical countries, are almost 

 entirely wanting. If we disregard the Malagasy sub-region, there 

 are scarcely forty species of land operculates on the whole African 

 continent. 



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 



j 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. 



I (a) The Senegamhian Province may be regarded as extending 



I from the mouth of the Senegal River to Cape Palmas. Only 

 i 8 genera of land MoUusca are known, including 4 Limicolaria 

 i and 3 Thapsia^ with 1 small Cyclophorus, Fresh-water genera 

 are abundant, and include most of the characteristic Ethiopian 

 forms. 



(h~) The West African Py^ovince extends from Cape Palmas to 

 1 the mouth of the Congo, and is rich in Mollusca. The great 

 ' Achatina^ largest of land snails, whose shell sometimes attains a 

 j length of 6 J in., Limicolaria^ Perideris, and Pseudachatina are 

 the characteristic forms. The Agnatha are represented by Ennea^ 

 :. Streptaxis^ and Streptostele. Rachis ^.nd Pachnodus, sub-genera 

 j of BuUminus, 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 Avion; Ustria, a slug with an external shell, 

 j akin to Parmacella ; and Aspidelus, a form intermediate between 

 j Melicarion and Limax. Claviger^ a handsome group akin to 

 j Cerithium^ is peculiar to the estuaries of West African rivers. 

 ( About sixteen species are known from the Oameroons District^ 



'' but no peculiar genera occur. The Freiich Congo District has 



