234 Quartaly Jotirnal of Conchology. 



to two very distinct regions — the Joutlicrn ha'f composing the 

 principal portion of the Caps ProvinLC, and the Tropical half 

 belonging to the Indo-Pacific region, or if that is too extensive an 

 area, to the subregion of East Africa and Madagascar. 



The principal ports are Zanzibar in S. Lat. 6°28', Mozambique 

 some 500 miles to the south, and Inhambane on the Tropic of 

 Capricorn separating the two provinces. The two next ports — 

 Delagoa Bay and Port Natal — are both v,-ithin 400 miles of the 

 tropics, whilst Algoa Bay, Mossel Bay and Cape Town are situated 

 towards the southern extremity of the Continent, 



The physical conditions of the two regions are very dissimilar. 

 In the one we have coral reefs, sloping sands and sheltered mud- 

 flats, a smooth sea, warm vrater and a comi)arative absence of sea 

 weed. In the other, steep exposed shores and primary rocks 

 clothed with abundance of sea weed, and continually beaten by a 

 stormy sea. Again, another difference is found in the tide, which 

 at Cape Town and Algoa Bay is but trifling, whilst at Mozambique 

 and Zanzibar the rise and fall is some 10 or 12 feet. 



Port Natal in its physical conditions, no less than in its geograph- 

 ical position stands as it v/ere, midway, between the two districts, 

 shewing, however, in the general character of its mollusca a more 

 decided alliance with the Cape than with the tropical East Coast. 



A most important agent in influencing the distribution of the 

 mollusca is to be found in the Mozambique current — a large body 

 of water from the Indian Ocean, which strikes the East Coast at 

 Cape Delgado, sweeps past Mozambique, Natal, &c., and finally 

 passes round the Cape of Good Hope. 



In regard to the mere number of species there is a similarity in 

 the two regions. When on the Coast, I collected about 392 

 species of Gastropoda ; of these 195 belong to the Cape region, 

 197 to the Coral seas. 



