890 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
the Sonoran of the surrounding plane by a belt or girdle of transition 
zone forms. 
The tropical region reaches the United States at two remote points— 
Florida and Texas. In the former it exists as a narrow sub-tropical 
belt encircling the southern half of the peninsula from Cape Malabar 
on the east to Tampa Bay on the west. In Texas it crosses the Lower 
Rio Grande from Mexico, and extends north to the Nueces River. In 
western Mexico the tropical region reaches Mazatlan. 
Fourteen families of tropical mammals inhabit North America north 
of Panama, namely: 
Didelphide. Dicotylide. Procyon‘de. Hapalide. 
Bradypodide. Tapiride. Solenodontide. Cebide. 
Myrmecophagide. Octodontide. Emballonuride. 
Dasypodide. Dasyproctidx. Phylostomatide. 
Of the above fourteen families, six reach the United States, namely, 
Didelphide, Dasypodide, Dicotylide, Procyonide, Emballonuride, and 
Phyllostomatide, and two of the latter (Didelphidw and Procyonide) 
penetrate the entire breadth of the Sonoran region, the Procyonide 
even entering the lower edge of the Boreal. Descending from families 
to genera, it is found that no less than sixty-two tropical genera of non- 
pelagic mammals inhabit North America north of Panama, of which 
number nine enter the United States from Mexico, namely, Didelphis, 
Tatusia, Dicotyles, Nasua, Procyon, Felis, Molossus, Nyctinomus, and 
Otopterus. Of these, Didelphis, Felis, and Procyon now reach consid- 
erably further north than the others, as just pointed out in speaking 
of the families to which they respectively belong. In explanation of 
this extended range it is found that these genera inhabited North 
America in pre-glacial times, and as a consequence have become aceli- 
matized to a wider range of climatic conditions. The semi-tropical 
belt of Florida is not known to possess any tropical mammals except 
bats and a large indigenous mouse (Sitomys macropus),* but it has not 
been explored by experienced mammal collectors. Still, its recent 
origin and complete isolation from other tropical areas would indicate 
the absence of terrestrial species derived from the south. At the same 
timeit isknown to berich in tropical plants, land shells, insects, and birds, 
as is shown in another part of the present paper (see post pp. 404, 
405). It contains nine genera of tropical birds, namely Zenaida, Geo- 
trygon, Starnenas, Rostrhamus, Polyborus, Crotophaga, Euetheia, Calli- 
chelidon, and Cereba. 
The following sixty-two genera of mammals belong to the North 
American tropical region. The nine preceded by the letter S enter 
the southern United States, which they penetrate varying distances. 
Nyctinomus and Otopterus inhabit the Lower Sonoran zone in common 
* Described by the writer as Hesperomys macropus in N, Am, Fauna, No. 4, Oct., 
1890, p. 53. 
