CHAP. XXII.] 
MOLLUSCA. 
523 
vertebrates. In the Helicidse, 10 genera are confined to the 
New, 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 all 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 Helicidfe 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 will evidently show which division is 
the most natural one for this group. The result is as follows : — 
Genera common to two or more Regions in, and confined to, each 
Primary Division of the Earth. 
Helioidte 
Operculata. 
Totals. 
Northern 
. 0 . 
. 0 . 
. 0) 
Southern 
. 0 . 
. 0 . 
. of 
Old World . 
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 
