CHAP. XXII.] MOLLUSCA. 623 



vertebrates. In the Helicidae, 10 genera are confined to the 

 i^ew, and 7 to the Old World, 16 being common to both. In the 

 Opercnlata the number of genera of restricted range is greater, — 

 the New World having 15, the Old World 32 genera, only 8 being 

 common to both. Of the New World genera 12 out of the 15 do 

 not occur at aU in South America ; and of those of the Old 

 World, 22 out of the 32 occur in a single region only. If we 

 take the northern and southern division proposed by Professor 

 Huxley (the latter comprising the Australian and Neotropical 

 regions), we find a much less well-marked diversity. Among 

 the Helicidae only 4 are exclusively northern, 8 southern ; while 

 among the Operculata 22 are northern, 16 southern. The best 

 way to compare these two kinds of primary division will be to 

 leave out all those genera confined to a single region each, and 

 to take account only of those characteristic of two or more of the 

 combined regions ; which wiU evidently show which division is 

 the most natural one for this group. The result is as follows : — 



Genera common to two or more Eegions in, and confined to, each 

 Primary Division of the Earth. 



Helicidae Operculata. Totals. 



(.Northern . . .0. . .0. . .0) 



( Southern . . . . . , . , . ) 



( Old World . . . 1 . . . 12 . . . 13 j 



\ New World . .4. . .0. . .4} 



We find then that the northern and southern division of the 

 globe is not at all supported by the distribution of the terrestrial 

 molluscs. It is indeed very remarkable, that the connection so 

 apparent in many groups between Australia and South America 

 is so scantily indicated here. The only facts supporting it seem 

 to be, the occurrence of Geotrochus (a sub-genus of Helix) in 

 Brazil, as well as in the Austro-Malayan and West Pacific Islands 

 and North Australia ; and of Bulimus in the same two parts of 

 the globe, but peculiar sub-genera in each. But in neither case 

 is there any affinity shown between the temperate portions of the 

 two regions, so that we must probably trace this resemblance to 

 some more ancient diffusion of types than that which led to the 

 similarity of plants and insects. Still more curious is the entire 



