in directions, and the influx
at certain points of great bodies of fresh water, limit the extension of
many species. Those which love deep water are arrested by shoals;
others, fitted for shallow seas, cannot migrate across unfathomable
abysses. The nature also of the ground has an important influence on the
testaceous fauna, both on the land and beneath the waters. Certain
species prefer a sandy, others a gravelly, and some a muddy sea-bottom.
On the land, limestone is of all rocks the most favourable to the number
and propagation of species of the genera Helix, Clausilia, Bulimus, and
others. Professor E. Forbes has shown as the result of his labours in
dredging in the AEgean Sea, that there are eight well-marked regions of
depth, each characterized by its peculiar testaceous fauna. The first of
these, called the littoral zone, extends to a depth of two fathoms only;
but this narrow belt is inhabited by more than one hundred species. The
second region, of which ten fathoms is the inferior limit, is almost
equally populous; and a copious list of species is given as
characteristic of each region down to the seventh, which lies between
the depths of 80 and 105 fathoms, all the inhabited space below this
being included in the eighth province, where no less than 65 species of
Testacea have been taken. The majority of the shells in this lowest zone
are white or transparent. Only two species of Mollusca are common to
all the eight regions, namely, _Arca lactea_ and _Cerithium lima_.[916]

_Great range of some provinces and species._--In Europe conchologists
distinguish between the arctic fauna, the southern boundary of which
corresponds with the isothermal line of 32 degrees F., and the Celtic,
which, commencing with that limit as its northern frontier, extends
southwards to the mouth of the English Channel and Cape Finisterre, in
France. From that point begins the Lusitanian fauna, which, according to
the recent observations of Mr. M'Andrew (1852), ranges to the Canary
Islands. The Mediterranean province is distinct from all those above
enumerated, although it has some species in common with each.

The Indo-Pacific region is by far the most extensive of all. It reaches
from the Red Sea and the eastern coast of Africa, to the Indian
Archipelago, and adjoining parts of the Pacific Ocean. To the geologist
it furnishes a fact of no small interest, by teaching us that one group
of living species of mollusca may prevail throughou