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 peculiar. 

 The molar tubercles are paired, though not always exacth^ 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. 



Fig. 09.— Bah'.mvs taylori. Skull, a. doilsal view; 6, ventra„ vikw; c, lateral view. 



Genus PEROMYSCUS Gloger (1841). 

 Peromysciis Gloukk, llaiul- u. ililfshudi d. Xatuif^cscli., I, 1841, pp. xxx, 9.5. 



Type. — -Peromyscus arboreus CJloger ( = Mus sijlvaticus novehoracen- 

 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 PEROMYSCUS FOUND ON THE 



MEXICAN BOUNDAKY LINE. 



a. Tail longer than head and body. 



b. Two front upper molars without .siib.sidiary cu.sps or corre.sponding enamel loops; soles 

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

 tip. 



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



cc. Length less than 220 mm. 



(/. Under surface gra3'ish white, stained with clay-color — never pure white. 



Peromyscus eremicus fraterculus (p. 433). 

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

 color (with or without a colored pectoral spot). 



aProc. U. S. Nat. Mus., XVI, 1S94, p. 758. 



