04: CONTRIBUTIONS TO NORTH AMERICAN ICHTHYOLOGY III. 



exilis, Noturus eleutltvrus by yotnrus mi urns, Mdauura prjfimcca by Mt- 

 lanura liini. 



XVI. Other species uuder similar circiirnstances have no such repre- 

 sentatives. Tlie case of Luxilns cornutus will ajjain 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 hctcrnro, 

 Poiiiolnhus cliri/socliloris^ and AnguUla vuhjaris. These species are now 

 abundant in Lake Michigan and Lake luie. iilthough formerly unknown 

 there. The range of certain Percirfa' and CentrarehuJiv has undoubtedly 

 been extended by the same means. 



XVIII. The characteristically American forms of lishes are, generally 

 si)eaking, rare or absent in the waters of New England and of the 

 Pacilic slo|)e. 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. Oi these, about 70 do not 

 occur in Xew Engl.ind (exclusive of Lake Champhiin, 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, Corcgotius, 

 Esox, i:)emotilns, llhinichthys, and possibly Aniiurus, are 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 s[)ecies. In the little White IJiver at Indianapolis, belonging 

 to the Mississippi basin, 70 species, representing 4S genera, are known 

 to occur — twice as many as inhabit all tln^ rivers of New England. 



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

 oi undisturbed stratified rocks or of glacial drift contains most genera 

 ami species. 



XX i. Conversely, rivers in regions of igneous or metamorphic rock 

 contain lewest'fepecies. 



XX IL Sources of streams on opposite sides of a high watershed often 

 have si)ecies in common which do not occur in the lower courses of tlie 

 saaic riviMs. The distribution of several mountain six-cies, as Sa'veliuH.s 

 fontiiKiUs and Ilijdropldox rubricroceus, will exemplif\' this. 



XX! II. Certain spe(>ies have a compact geographical range, occurring 

 in all the rivers within this range, without apparent regard to the<lire<'- 

 tion (»f their tlow. Sncli aie Lepiopomus obaatms in the Alabama, Ten- 



