366 THE SONORAN REGION. [CHAP. 



Onychomys and Oryzomys are restricted. Yet their near alliance 

 to the Old World hamsters indicates that the group must have 

 had a northern origin, although the genus may have attained 

 its present distinctive features within the Sonoran area. The 

 genus Sigmodon, which differs from the last in the pattern of the 

 molar teeth, and is represented solely by the rice-rat, does not 

 range north of the Sonoran region, although extending into South 

 America as far as Ecuador. The wood-rats (Neotoma), in which 

 the molars simulate the prismatic appearance of those of the voles, 

 .are also largely Sonoran, although they extend into the Canadian 



FIG. 72. FACE OF Geomys bursarius, SHOWING GROOVED UPPER INCISORS 



AND OPENINGS OF CHEEK-POUCHES. 



FIG. 73. FACE OF 7"komomys talpoides, SHOWING SMOOTH UPPER INCISORS 



AND OPENINGS OF CHEEK-POUCHES. 



sub-region of the Holarctic, where they are represented by a 

 distinct sub-genus (Teonoma). The round-tailed musk-rat of 

 Florida (Neofiber) is an exclusively Sonoran type, although it is 

 regarded by Dr Merriam merely as a sub-genus of Microtus. 

 Highly characteristic of the region are the pouched rats, consti- 

 tuting the genera GeomyidcR. Of these, the typical genus Geomys * 

 extends northwards into the Transition zone and southwards into 

 Central America ; while the nearly-allied Thomomys> in which the 

 upper incisor teeth are smooth instead of grooved, penetrates 

 into the Canadian sub-region of the Holarctic, although unknown 



1 Subdivided into eight genera by Merriam, North American Fauna, Part 

 viii., Washington (1895). 



