382 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



head and body, 53 mm.; tail vertebrae, 32; hind foot, without claw, 

 13; ear above crown, 5. Skull, 18 by 10. Coloration somewhat 

 similar to that of Mus musculus, but without the salmon tint beneath. 

 The upper surface is brownish gray and the under parts grayish 

 white. The tail is scantily haired, and not distinctly bicolor. 



Cranial and dental characters. — In addition to the generic charac- 

 ters pointed out by Dr. F. W. True," the dentition is also pecuUar. 

 The niolar tubercles are paired, though not always exactly opposite. 

 As they are ground down with wear, the enamel pattern of the 

 grinding surfaces assumes almost a lozenge shape (fig. 68). The 

 skull also has an unusual shape, which is shown in fig. 69. 



a b c 



Fig. 09.— Baiomys taylori. • Skull, a, dorsal view; 6, ventral view; c, lateral view. 



Genus PEROMYSCUS Gloger (1841). 

 Perorriyscus Glooer, Hand- u. Hilfsbuch d. Naturgesch., I, 1841, pp. xxx, 95. 



Type. — Peromyscus arboreus Gloger { = Mus sylvaticus noveioracen- 

 sis Fischer). 



Characters. — Form slender; tail not shorter than body without 

 head; ears large; claws weak; hind legs and feet long, the latter with 

 six conical tubercles; soles naked or less than half hairy; skull with 

 upper margin of orbit sharp, but without a bead; mandible with 

 coronoid process short; dental tubercles low. 



SYNOPTICAL KEY TO THE SPECIES AND SUBSPECIES OF PEKOMYSCUS FOUND ON THE 



MEXICAN BOUNDARY LINE. 



a. Tail longer than head and body. 



b. Two front upper molars without subsidiary cusps or "corresponding enamel loops; soles 

 of feet naked to the heel; tail nearly naked at base, without a distinct brush at the 

 tip. 



c. Length more than 220 ram Peromyscus califomicus insignis (p. 429). 



cc. Length less than 220 mm. 

 d. Under surface grayish white, stained with clay-color — never pure white. 



Peromyscus eremicus fralerculus (p. 433). 

 dd. Under surface (usually) pure white, or (rarely) beautifully tinted with salmon- 

 color (with or without a colored pectoral spot). 



aPrco. U. S. Nat. Mus., XVI, 1894, p. 758. 



