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CHAP. XV.] THE NEARCTIC REGION. 125
over the United States. Species oh^itrina, Zonites, Pupa, and
Succinea, are found in Greenland ; aM Eastern Palsearctic species
of Vitrina, Patula, and Pupa occur in Alaska. More than 30
species of shells living in the Eastern States, are found fossil
in the Post-Pliocene deposits of the Ohio and Mississippi.
Fresh-water Shells.—North America surpasses every other part
of the globe in the number and variety of its fresh-water moUusca,
both univalve and bivalve. The numbers up to 1866 were as
follows :—Melaniadse, 380 species ; Paludinida3, 58 species
Cycladidse, 44 species ; and Unionidse, 552 species. The last
family had, however, increased to 832 species in 1874, according
to Dr. Isaac Lea, who has made them his special study ;
;
but it
is probable that many of these are such as would be considered
varieties by most conchologists. Many of the species of Unio are
very large, of varied forms, and rich internal colouring, and the
gToup forms a prominent feature of the Nearctic fauna. By far the
larger proportion of the fresh-water shells inhabit the Eastern or
Alleghany sub-region ; and their great development is a powerful
argument against any recent extensive submergence beneath the
ocean of the lowlands of North America.
The Nearctic Sub-regions.
The sub-divisions of the Nearctic region, although pretty
clearly indicated by physical features and peculiarities of
climate and vegetation, are by no means so strongly marked
out in their zoology as we might expect. The same genera, as
a rule, extend over the whole region ; while the species of the
several sub-regions are in most cases different. Even the vast
range of the Eocky Mountains has not been an effectual barrier
against this wide dispersal of the same forms of life ; and
although some important groups are limited by it, these are
exceptions to the rule. Even now, we find fertile valleys and
plateaus of moderate elevation, penetrating the range on either
side ; and both to the north and south there are passes which
can be freely traversed by most animals during the summer.
Previous to the glacial epoch there was probably a warm period,
when every part of the range supported an abundant and varied