MAMMALS OF THE MEXICAN BOUNDARY. 



445 



Description.' -Smallest of the desert mice. Length of hind foot, 

 19 mm.; ear from notch, 16. Skull, 24 by 12. Color above, drab 

 gray, thickly mixed with black; sides ochraceoiis cinnamon; under 

 surfaces and feet white; ears purplish black, almost naked; tail 

 quite densely coated with blackish hairs. 



Cranial and dental characters. — Compared with Peromyscus eremi- 

 cus (Baird), its skull (fig. 107) is smaller, broad interorbitally, higher 

 posteriorly, narrower and more depressed anteriorly. The audital 



P'iG. 107.— Peromyscus tibueonensis. Skull, a, dorsal view; 6, ventral view; c, lateral 



VIEW. 



bullae are much less developed, and the incisive foramina and inter- 

 pterygoid fossa wider. The teeth (fig. 108) are considerably smaller. 

 Remarks. — This species was taken on Tiburon Island by Mr. J. W. 

 Mitchell, who accompanied Doctor McGee, of the Bureau of Ethnology, 

 on his exi^loration in the region inhabited by the 

 Seri Indians during the season of 1895-96. 

 Though occurring beyond the scope embraced by 

 the present report, the species is here introduced 

 for the purpose of completing the history of the 

 desert mice that are closely related to Peromyscus 

 eremicus. This insular form was evidentl}^ de- 

 rived from P. eremicus of the adjacent mainland, 

 from which it differs in being smaller, with a 

 darker coloration, more hairy tail, and in having 

 certain cranial peculiarities, described above. 



Fig. 108.— Peromyscus ti- 

 BURONENSis. Crowns of 



MOLAR TEETH. O, LOWER 



series; b, UPPER series. 



Genus SIGMODON Say and Orel (1825). 



Sigmodon Say and Ord, Journ. Acad. Nat. Sci. Pliila., IV, Pt. 2, 1825, p. 352. 



Dentition. — I. jEi; M. ^ = 16. 



Type. — Sigmodon hispidus Say and Ord. 



Characters. — Form ratlike, with prominent ears and a somewliat 

 hairy tail (fig. 109). Grinding teeth, with crowns flattened and 

 divided into S-shaped loops formed by plates of hard enamel inclos- 

 ing a softer substance (dentine); skull with a prominent supraorbital 

 fronto-parietal ridge. 



