87 



number being thirty-six ; while ninety-six species of birds tlo not wander 

 north of it. To this category many, of the common species of the Mid- 

 dle States belong. 



North of this the species of vertebrates are circumpolar or arctic. 



The ichthyological fauna of the two Eastern subregious remains to 

 be considered. For the present, they will be united, though the distri- 

 bution of fresh-water fishes is governed by laws similar to those con- 

 trolling terrestrial vertebrates and other animals, in spite of the 

 seemingly confined nature of their habitat. With this general principle 

 in view, we may revert briefly to this distribution over this district of 

 the Nearctic region. This large area is characterized by the distribution 

 of several species in all its waters, or nearly so, so far as jet exam- 

 ined — those of Semotilus, Ceratichthys, Hy2)silepis, Catostomns, etc., or 

 by the universal recurrence of the same in suitable situations ; and by 

 the representation of these and other genera by nearly allied species in 

 its different portions. The fauna of the tributaries of the Mississippi 

 constitutes, it might be said, that of our district; while the slight vari- 

 ations presented by the Atlantic coast streams might be regarded as 

 exceptional. The fauna of the great lakes combines the peculiarities of 

 both, possessing as a special peculiarity, (1), which belongs to the Lake 

 region, which, in the district, commences at latitude 42° and extends to 

 the Arctic regions, the range of the genus Coregonus. Tlie peculiarity 

 of the Atlantic subdistrict (II) may be said to be the abundance of Esox, 

 Salmo, and Anguilla, and the absence of Haploidonotus. The first two 

 are abundant in the Lake region, while AnguiUa and Haploidonotus have 

 but a partial distribution there. In (111), the Mississippi basin, Esox is 

 represented by but few species, and remarkably few individuals. Salmo 

 occurs abundantly in the upper parts of the Missouri tributaries, exists 

 in the western mountain-streams of the Alleghanies, becoming rare in 

 those of the Kanawha, and only occurring near the highest summits ia 

 those of the Tennessee, south to the line of South Carolina. It is 

 especially characterized by the paddle-fish [Spatularia or Polyodon), the 

 shovel-sturgeon {Scaphirhyjichops), and the alligator-gar [Atractostens) ; 

 also by the buffalo [BubaUchthys), the Cycleptus, etc., among suckers, 

 and the fork-tailed catfish (Ichthaelurus). Among Percomorphs, the Haj)- 

 loidonotus is the characteristic genus ; and among those allied to the 

 herring, the genus Hyodon. Numerous species are confined to its afflu- 

 ents. The gradation from the Mississippi grouping of species to the 

 Atlantic is very gradual, and takes place in successional order from 



