26 Merriam — Geograpliic Dlstributiou of Life. 



The SoNORAx\ Region as a whole stretches across the continent 

 from Atlantic to Pacific, covering nearly the whole country south 

 of latitude 43° and reaching northward on the Great Plains and 

 Great Basin to about latitude 48°. It is invaded from the north 

 by three principal intrusions of Boreal forms along the three 

 great mountain systems already mentioned ; while to the south- 

 ward it occupies the great interior basin of Mexico and extends 

 into the troj^ics along the highlands of the interior. It covers 

 also the peninsula of Lower California, the southern part of 

 : which seems entitled to rank as an independent subdivision. 

 The following genera belong exclusively to the Sonoran Region 

 (as distinguished from the Boreal), none of them ranging north 

 beyond the Transition Zone. Those preceded by the letter T are 

 intrusions from the Tropical Region. 



T Didelphis Bassariscus 



T Tatusia T Nasna 

 T Dicotyles Conepatus 



Reithrodontoniys * Spilogale 



Onychomys Notiosorex 



Oryzomys Scalops 



Sigmodon Corynorliinus 



Geomys Euderma 



Dipodomys Antrozous 



PerodiiHis f Nycticejiis 



Microdipodops T MoIoshus 



Perognatlius T Nyctinomus 



Heteromys 2' Otoptcrus 

 Urocyon 



In addition to the al)Ove, 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 : 



*The generiii ivcime Relthrodontomys was proposed by Giglioli in 1873 

 (Richerche iiitorno alia Distribuzione Geografica Generale, Roma, 187o, 

 p. 11)0), and antedates OcJietodun of Coues. 



fThe generic name Perodipus was proposed in ISO? l)y Fitzinger for the 

 five-toed kangaroo rats (Hitzungsber. math. nat. Glasse, K. Akad. Wiss. 

 Wien, LVI, 1867, ]). 12()), thus antedating by twenty-three years the name 

 Dipodops proposed by the writer for the same type in 1890 (North Am. 

 Fauna, No. 3, Sei)tember, 1890, p. 72;. Both generic names were based on 

 DipudiiiiiiiH fiijilis of Gambel, from Los Angeles, California. 



