GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 391 
with the tropical; Didelphis pushes completely through the humid 
division of the Sonoran region; and Felis and Procyon enter the lower 
edge of the boreal. 
NORTH AMERICAN TROPICAL GENERA, 
Chironectes. Stenoderma. Noctilio. Mimon. 
S$ Didelphis. Chiroderma. S Molossus. Hemiderma. 
Bradypus. Pygoderma. S Nyctinomus. Glossophaga. 
Cholcepus. Sturnira. ; Chilonycteris. Phyllonycteris. 
Myrmecophaga. Brachyphylla. Mormops. Monophylla. 
Tamandua. S Felis. Centurio. Leptonycteris. 
Cycloturas. S Procyon. Desmodus. Glossonycteris. 
S Tatusia. Bassaricyon. Diphylla. Choeronycteris. 
S Dicotyles. S Nasua. Midas. Artibeus. 
Elasmognathus. Cercoleptes. Mycetes. Vampyrops. 
Capromys. Galictis. Lonchorhina. Chrysothrix. 
Plagiodontia. Solenodon. S Otopterus, Nyctipithecus. 
Echinomys. Natalus. Vampyrus. Ateles. 
Synetheres. Rhynchonycteris. Micronycteris. Cebus. 
Dasyprocta. Saccopteryx. Trachyops. 
Celogenys. Diclidurus. Phyllostoma. 
Recapitulating, it is found that of the 134 genera of non-pelagic 
mammals inhabiting North America north of Panama, 53 are exclu- 
sively tropical, 20 exclusively Sonoran, and 20 exclusively boreal. In 
addition 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: Tropical, 62; Sonoran, 34; boreal, 31; thus 
leaving but 7 genera out of a total of 134 that are not distinetly refer- 
able 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, Spermophilus, Lepus, Canis, and Lutra, each of which ranges 
over large parts of both boreal and Sonoran regions. All except Sper- 
mophilus inhabit the tropical region also, and all are of great antiquity, 
as will be shown presently (p. 393). The genera Spermophilus 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 Sciurus might 
be considered tropical and Sonoran. 
Omitting Mexico and Central America, and regarding the 9 intru- 
sive tropical genera already mentioned as Sonoran (in contradistine- 
tion to boreal),it is found that 81 genera of non-pelagic mammals inhabit 
the United States and Canada, of which 45 may be looked upon as of 
Sonoran origin and 51 as of boreal origin. The 7 genera remaining are 
those mentioned in the last paragraph. 
