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526 GEOGRAPHICAL ZOOLOGY. [part iv.
distribution of these animals is the geological antiquity of the
group, and the amount of change exhibited in time, by species
and genera. Now we find that most of the genera of land-shells
range back to the Eocene period, while those inhabiting fresh
water are found almost unchanged in the Wealden. In North
America a species of Pupa and one of Zonitcs, have been dis-
covered in the coal measures, along with Labyrinthodonts ; and
this fact seems to imply, that many more terrestrial molluscs
would be discovered, if fresh-water deposits, made under favour-
able conditions, were more frequently met with in the older
rocks. If then the existing groups of land-molluscs are of such
vast antiquity, and possess some means, however rarely occurring,
of crossing seas and oceans, we need not wonder at the wide and
erratic distribution now presented by so many of the groups
and we must not expect them to- conform very closely to those
regions which limit the range of animals of higher organization
and less antiquity.
The total number of species of pulmoniferous mollusca is about
7,000, according to the estimate of Mr. Woodward, brought down
to 1868 by Mr. Tate. But this number would be largely increased
if the estimates of specialists were taken. Mr. Woodward
for example, gives 760 as the number of species in the West
Indian Islands ; whereas Mr. Thomas Bland, who has made the
shells of these islands a special study, considers that there were
1,340 species in 1866. So, the land-shells of the Sandwich
Islands are given at 267; but Mr. Gulick has added 120 species
of Achatinellidse, bringing the numbers up to nearly 400,— but
no doubt several of these are so closely related that many con-
chologists would class them as varieties. The land-shell fauna
of the Antilles is undoubtedly the most remarkable in the world,
and it has been made the subject of much interesting discussion
by Mr. Bland and others. This fauna differs from that of aU
other parts of the globe in the proportions of the operculate to
the inoperculate shells. The Operculata of the globe are about
one-seventh, the Inoperculata about six-sevenths of the whole ;
and some general approximation to this proportion (or a much
smaller one) exists in almost all the continents, islands, and
;