- Numerical Rccapiiulatlon — Genera of 3Jammals. 35 



Stenoilcrnui Ceiiturio Chrysothrix 



Chiroderma Desmodus Nj^ctipithecus 



I'\'goderina Diphylla Ateles 



Sturnira Midas Cebus 



Bracliyphylla Mycetes 



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

 four genera of non-pelagic mammals inhabiting North America 

 north of Panama, fiftj^-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 ])roper 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 (St/naptomys) 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, Cdiii'i, and Liitra, each of which ranges over large 

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

 phllus inhal)it the Tropical Region also, and all are of great an- 

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

 philus 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 Sciunis might be considered TrojDical 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. 



