QiiarUrJy J^MKial of Coiclo'ogy. 235 



When, however, the species and genera are condderei t'lere is 

 a striking difference. 



Of the entire 197 tropical species I did not find one at Cape 

 Town and at Algoa Bay, but a few examples of one species {Akrita 

 albiccUa, L.) 



Tiiis can only be explained on the supposition that the physical 

 conditions of '.he latter places are incompatible with the welfare of 

 tropical specie^, as otherwi.e the Mo2ambi;^ue Current would 

 unJj'jbtcdly bring down numerous species. At Natal, where 

 conditions aie more favourable, cut of 85 species collected 33 

 were also found in tl.e Indian Ocean. It is interesting to notice 

 tlial the Cape species proper, do not extend beyond the limits of 

 the Province, less than half a dozen being found at Inhambane, 

 immediately within the tropics. 



As might be expected the relative preponderance of the carni- 

 vorous and i)hytophagous gastropoda of the two regions is reversed. 

 Of 214 species of Siphonostomata, 86 are found to the south of 

 the tropics and 128 within the tropics. Of 166 species of 

 Holostomata, 105 are found in the South and 61 within the tropics. 

 At Natal the numbers are about equally balanced, 43 Siphon- 

 ostomata to 42 Holostomata. Certain genera are confined to 

 each province, c. g. Plcroco os, Terebni, Turbinella, Hipponyx, 

 Umbrella are characteristic of the tropical parts, as are also Aura, 

 Aplysia, &-r., v.hilst Cominella, Haliotis, Calyptrxa, Crepidiila 

 and others are found only in the Cape region. 



Again, certain genera are distinguished by possessing a greater 

 number of species in one province than in another by being in- 

 dividually more abundant, or by the specits being more typical. 



In South Africa the genus Patella (including subgenera) 

 furnishes some 30 species; at Mozambi.|U; only two are found, 

 of which one extends down to Natal. 



