87 



iiniiiber being thirty-six ; while ninety-six species of birds do not wander 

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

 dle States belong. 



Xorth of this the species of vertebrates are eircumpolar or arctic. 



The iclithyological fauna of the two Eastern subregions 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 vii'w, we may revert briefiy to this distribution over this district of 

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

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

 iiiotl — those of Scmoiilus, Cerat i chili jj.s, Ilmhsilrpi.s, Catostomus, 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 diflerent 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, (I), 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. The peculiarity 

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

 SaJmo, and Anguilla, and the absence of Ihiploidonotus. The first two 

 are abundant in the Lake region, while AnfjulUa and IlaploUJonotm have 

 but a partial distribution there. In (III), the Mississippi basin, Eftox is 

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

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

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

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

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

 especially characterized by the paddle-fish [Spatularla or Polijodon)^ the 

 shovel-sturgeon {Scaphirhynchops), and the alligator-gar {Atmctosteus) ] 

 also by the buffalo {Buhalichthys), the Cyclcptus, etc., among suckers, 

 and the fork-tailed catfisib. {IcUthaelurus). Among Percomorphs, the Ha})- 

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

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

 euts. The gradation from the Mississipi)i grouping of species to the 

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



