CHAP. XI CEXrRAL 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 

 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. 



(«) The Senegamhian Province ma}^ be regarded as extending 

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

 8 genera of land Mollusca are known, including -i Limicolaria 

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

 are abundant, and include most of the cliaracteristic Ethiopian 

 forms. 



(6) 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 6 1 in., Limicolaria^ Perideris, and Pseudachatina are 

 the characteristic forms. The Agnatha are represented by Eyinea^ 

 Streptaxis^ and Streptostele. Rachis and Paclinodus^ sub-genera 

 of Bidiminns^ occar also on the east coast. A special feature is 

 the develoj^ment 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 Caiyieroons District^ 

 but no peculiar genera occur. The French Congo District has 



