Numerical Ilccup it a hit Ion Genera of Mammals. 35 



i 



Stenodeniui Centurio Chrysothrix 



(Miiroderma Dcsmodus Nyctipithecus 



Pygoderma Diphylla Ateles 



Sturnira Midas Cebus 



Brachyphylla Mycetes 



Recapitulating, it is found that of the one hundred and thirty 

 four genera of non-pelagic mammals inhabiting North America 

 north of Panama, fifty-three are exclusively Tropical, twenty 

 exclusively Sonoran, and twenty exclusively Boreal. In addi- 

 tion to these genera, which do not outstep the limits of the 

 regions to which they severally belong, a number of others are 

 clearly referable to the same regions, though ranging varying 

 distances beyond their proper boundaries. Including these 

 genera, the number belonging to each region is as follows : Tropi- 

 cal, sixty-two ; Sonoran, thirty-four ; Boreal, thirty-one thus 

 leaving but seven genera out of a total of one hundred and thirty- 

 four that are not distinctly referable to one of the three regions. 

 One of these (Synaptomys) is not known to occur outside the 

 limits of the Transition Zone, leaving but six genera that have not 

 been assigned. These genera are Sciuropterus, Sciurus, Spermo- 

 philus, Lepus, Canis, and Lutra, each of which ranges over large 

 parts of both Boreal and Sonoran Regions. All except Spermo- 

 philas inhabit the Tropical Region also, and all are of great an- 

 tiquity, as will be shown presently (p. 37). The genera Spenno- 

 philu-s and Lepus might be referred to the Sonoran Region because 

 the great majority of their species are confined to it; and for 

 the same reason Sdurus might be considered Tropical and Sono- 

 ran. 



Omitting Mexico and Central America, and regarding the nine 

 intrusive Tropical genera already mentioned as Sonoran (in con- 

 tradistinction to Boreal), it is found that eighty-one genera of 

 non-pelagic mammals inhabit the United States and Canada, of 

 which forty -three may be looked upon as of Sonoran origin and 

 thirty-one as of Boreal origin. The seven genera remaining are 

 those mentioned in the last paragraph. 



