412 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
distinct genera; the jumping mouse (Xapus |[=Zapus|) is a peculiar 
form of the jerboa family; and then we come to the pouched rats (Geo- 
myide), a very curious family consisting of four genera and nineteen 
species, peculiar to North America, though not confined to the Neare- 
tic region. The prairie dogs (Cynomys), the tree porcupine (Hrethizon), 
the curious sewellel (Haploodon [=Aplodontia]), and the opossum 
(Didelphis) complete the list of peculiar mammalia which distinguish 
the northern region of the New World from that of the Old.” (Island 
Life, p. 48.) 
As already shown in an earlier part of the present essay, most of 
these genera and several of the families belong to the austral or 
Sonoran region and have no place in the Boreal fauna—the only one 
that can be compared with the fauna of northern Eurasia. As a mat- 
ter of fact, eighty-one genera of non-pelagic mammals are now recog- 
nized in “extra-tropical” North America—the so-called Nearctie region. 
Of this number forty-one are found in no other part of the world.* 
These genera are enumerated in the following table, which brings out 
the important fact that no less than thirty-two, or 78 per cent, are of 
Sonoran or austral origin, while only nine, or 22 per cent, are of Boreal 
origin. Of these nine genera now confined to North America, Ovibos 
inhabited polar Eurasia in Pleistocene times; Neurotrichus is not recog- 
nized by Flower and Lydekker as more than sub-generically separable 
from Urotrichus of Japan, and Synaptomys is not known except from 
the Transition zone of the United States and is here classed as Boreal 
because of its close relationship to the trans-continental Boreal genus 
Myodes. Omitting these three, Boreal North America has but six 
genera of mammals not known from Boreal Eurasia. 
Peculiar genera of mammals inhabiting North America north of Mexico. 
OF BOREAL ORIGIN. 
Mazama. Fiber. Zapus. Neurotrichus. 
Ovibos. Synaptomys. Erethizon. Condylura. 
Aplodontia. 
OF SONORAN ORIGIN. 
Cariacus. Neotoma. Urocyon. Scapanus. 
Antilocapra. Thomomys. Bassariscus. Blarina. 
Cynoniys. Geomys. Taxidea, Antrozous. 
Reithrodontomys. Dipodomys. Conepatus. Nycticejus. 
Sitomys. Perodipus. Mephitis. Otopterus. 
Oryzomys. Microdipodops. Spilogale. Corynorhinus. 
Onyechomys. Perognathus. Notiosorex. Kuderma. 
Sigmodon. Heteromys. Scalops. Atalapha. 
* The intrusive genera Didelphis, Tatusia, Dicotyles, Procyon, Nasua, and Molossus, 
which are clearly of South American origin, are not here included. 
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