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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
Operciilata 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 Helicidse 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 foUows :
Genera common to two or more Regions in, and confined to, each
Primary Division of the Earth.
(Northern . . .0.
Helicidae Operculata. Totals.
. .0. . .0)
\ Southern . . . . . . . . .Of
( Old World .
New World I
.
.
. 1 .
.4.
.
.
. 12 .
.0.
.
.
. 13 )
.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 BJimus 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