ee 
GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 385 
In addition to the above, the following genera seem to be of Sonoran 
or austral origin, although reaching and in some cases penetrating a 
considerable distance into the boreal region: 
Cariacus, Neotoma. Mephitis. Blarina. 
Antiloeapra. Thomomys. T Felis. Atalapha. 
Cynomys. T Procyon, Lynx. Vesperugo. 
Sitomys.* Taxidea. Scapanus. Vespertilio. 
The genera Sitomys, Mephitis, Lynx, Atalapha, Vesperugo, and Ves- 
pertilio range well north in the Boreal zone, where each is represented 
by a single species. In the Sonoran zone, on the other hand, these 
Same genera reach their maximum development and are represented by 
numerous species. 
Besides the genera above enumerated, a number of sub-genera belong 
to the Sonoran region. Among these are Neosciurus and Parasciurus 
(subgenera of Sciurus), Xerospermophilus,t Ammospermophilus,t and 
Letidomys (sub-genera of Spermophilus), Pitymys, Pedomys, and Neofiber 
(sub-genera of Arvicola), and Chetodipus (a sub-genus of Perognathus, 
which is almost entitled to rank as a full genus). 
The Sonoran region may be divided by temperature into two princi- 
paltrans-continental zones, (a) Upper Sonoran, and (b) Lower Sonoran; § 
and each of these in turn may be sub-divided into arid and humid 
divisions. 
The gray fox, Urocyon, ranges over both Upper and Lower Sonoran 
from Atlantic to Pacific; and pocket gophers of the genus Geomys 
inhabit both these divisions on the Great Plains and in the Mississippi 
Valley, and range east to the Atlantic in the Austro-riparian zone. 
30th divisions of the Lower Sonoran are inhabited by the trans-con- 
tinental genera Reithrodontomys, Sigmodon, Corynorhinus, Nyctinomus, 
Otopterus, Neotoma, and Spilogale, though in the west the two last men- 
tioned range through the Upper Sonoran also. 
The humid Lower Sonoran or Austro-riparian is a division of much 
importance. It begins on the Atlantic seaboard at the mouth of Ches- 
apeake Bay and stretches thence southwesterly, embracing the alluvial 
lands of the South Atlantic and Gulf States below what geologists 
*The generic name Hesperomys being untenable, Allen has recently substituted 
for it the name Vesperimus proposed by Coues as a subgenus in 1874 (Bull, Am. Mus. 
Nat, Hist., June, 1894, m1, No. 2, pp. 291-297). Vesperimus is ante-dated by Sitomys 
of Fitzinger, proposed in 1867, and based on Gapper’s Cricetus myoides from Lake 
Simcoe, Ontario, Canada (Sitzungsber. math. nat. Classe, K. Akad. Wiss. Wien. 1867, 
LVI, p. 97). -Gapper’s Cricetus myoides is the common white-footed mouse of southern 
Ontario and northern New York, which therefore becomes the type of the genus. 
t Xerospermophilus, sub-gen. noy., proposed for Spermophilus mohavensis (type) and 
the allied species of the S. spilosoma group. 
t Ammospermophilus, sab-gen. noy., proposed for Spermophilus lewcurus (type) and 
allied species. 
\ The great Lower Sonoran Zone may be split lengthwise (in an east and west 
direction) into two belts which have not yet been thoroughly differentiated. 
H, Mis, 334, pt. 1 
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