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 



