LAND REGIONS. 189 



but the bivalves are principally Spatha, Iridina, and the fresh- 

 water oyster, ^theria. The Corbiculadae are represented by 

 the genus Cyreuella and two species of Corbicula. There 

 are two African species of Dreissensia ; both smaller than the 

 D. poIymo7-pha, which has spread with such amazing rapidit}^ 

 throughout Europe. 



In Lake Tanganyika occur two special genera, viz. : Tiphobia, 

 somewhat resembling the American genus lo, and perhaps a 

 Melanian, and jSTeothauma, a group of the Paludinidaj ; besides 

 some mollusks simulating marine genera, such as Trochus, 

 Littorina, etc. A number of species of Melania have been 

 described from Lake Nyassa, but I think that they are mostly 

 extreme forms of 31. tuberculata, Miiller. This lake contains 

 several Lanistes, Paludina, Physa, etc., with a Unio and two 

 Spathas. 



The few African C3'clostoma3 mostly occur upon the East 

 Coast, Mozambique, Zanzibar, etc. 



8. West African Region. 



The malacological fauna of intertropical West Africa appeal's 

 to be very rich, although the country is far from being thoroughly 

 explored. With such characteristic genera as Columna, Peri- 

 deris, Pseudachatina, Streptostele, Galatea, Fischeria, there are 

 associated others which indicate relationship with Central Africa; 

 these are Achatina, with equally large species, Ennea, and 

 particularly the fiuviatile genera Ampullaria, Lanistes, Isidora, 

 Physopsis, Iridina, Spatha. Only two operculated land snails 

 are known. The river mouths abound with brackish-water shells 

 T_ympanotomus, Capsella. 



In Gruinea, Morelet enumerated in 1868, 141 species, of which 

 were Helix 6 (mostly of the group Nanina), Streptaxis 5, 

 Limicolaria T, Achatina 7, Perideris 10, Stenogyra 3, Ennea 5, 

 Melania 5, Pirena 3, Neritina 4, Unionidse 5, etc. 



In the Gabon region about 50 species are catalogued also by 

 Morelet. The characteristic genus is Pseudachatina, including 5 

 species. Some of the largest Achatinse are found in this district, 

 but the metropolis of this genus is in Angola and Benguela, from 

 which 16 species have been sent. Here the fresh-water pulmonates 

 are unexpectedl}^ largely developed ; there are 3 Planorbis, 4 

 Limnsea, 10 Phj'sa. 



The little, mountainous Prince's Island aflbrds 25 species ; its 

 specialties being the genera Columna, 3 species, and Streptostele, 

 4 species. 



The Island of St. Helena is 1200 miles from the nearest African 

 coast of Benguela. It is entirely volcanic. The indigenous 

 plants are all peculiar, and not more related to those of Western 

 Africa than to Brazil. The land shells are all peculiar, except a 



