CHAP. XV.] 
THE NEARCTIG REGION. 
125 
over the United States. Species of Vitrina, Zonites, Pupa, and 
Succinea, are found in Greenland ; and Eastern Palaearctic 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 mollusca, 
both univalve and bivalve. The numbers up to 1866 were as 
follows : — Melaniadse, 380 species ; Paludinidm, 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 TJnio are 
very large, of varied forms, and rich internal colouring, and the 
group 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 
