94 CONTRIBUTIONS TO NORTH AMERICAN ICHTHYOLOGY III. 



exilis, Noturus eleutlierus by Noturus tniurus, Melanura pygmcea by Me- 

 lanura, limi. 



XVI. Other species under similar circumstances have no such repre- 

 sentatives. The case of Luxllus cornutus will again illustrate. 



XVII. Certain species have been known to extend their geographical 

 range since the opening of the canals. Such are more especially the 

 migratory species of probably marine origin, as Dorosoma heterura, 

 Pomolobus chrysochloris, and Anguilla vulyaris. These species are now 

 abundant in Lake Michigan and Lake Erie, although formerly unknown 

 there. The range of certain Perciclce and Centrarchidce has undoubtedly 

 been extended by the same means. 



XVIII. The characteristically American forms of fishes are, generally 

 speaking, rare or absent in the waters of New England and of the 

 Pacific slope. This fact has been well stated by Professor Agassiz, who 

 called New England " a zoological island". 



About 105 genera of fresh-water fishes occur in the waters of the 

 United States east of the Mississippi River. Of these, about 76 do not 

 occur in New England (exclusive of Lake Champloiu, the fauna of 

 which is nearly identical with that of Lake Ontario.) Of these 30 or 

 fewer -genera occurring in New England, all but Salvelinus^ Coregonus, 

 Esox, SemotiluS) Rliiniolitliys, and possibly Amiurm, tire represented by 

 a single species each. From 30 to 35 genera occur in the waters of the 

 Pacific slope. 



XIX. The larger the river-basin, the greater its variety of forms, both 

 genera and species. In the little White Kiver at Indianapolis, belonging 

 to the Mississippi basin, 70 species, representing 48 genera, are known 

 to occur tewice as many as inhabit all the rivers of New England. 



XX. Other things being equal, a river whose course lies in a region 

 ot undisturbed stratified rocks or of glacial drift contains most genera 

 and species. 



XXI. Conversely, rivers in regions of igneous or inetamorphic rock 

 contain fewest species. 



XXII. Sources of streams on opposite sides of a high watershed often 

 have species in common which do not occur in the lower courses of the 

 same rivers. The distribution of several mountain species, as Salvelinus 

 fontinalis and Hydrophlox rubricroceus, will exemplify this. 



XXIII. Certain species have a compact geographical range, occurring 

 in all the rivers within this range, without apparent regard to the direc- 

 tion of their flow. Such are Lepiopomus obscurus in the Alabama, Ten- 



