364 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



Hind foot (fig. 59) 6-tuberculate; tubercles paired. Pollex with a 

 short nail in place of a claw. No cheek-pouches. Tail long, nearly 

 naked, with rings of overlapping scales. Vertebrae: C 7, D 13, L 6, 

 S 4, C 26-32. {Flower and LydeMer.) 



This genus is represented by four very distinct species on the Mexi- 

 can Boundary Line. 



KEY TO THE SPECIES OF MUS FOUND ON THE BOUNDARY LINE. 



u. Size large. Length mdre than 300 mm. 



h. Tail about as long as liead and body, dusky above and pale below; color of body gray- 

 ish-brown above, soiled grayish-white below Mus norvegieus (p. 364). 



hb. Tail considerably longer than head and body, and dusky all round. 

 c. Color reddish-brown above, and white below; pelage mixed with spines. 



Mus alexandrinus (p. 366). 



cc. Color blue-black, darker on the back, more slaty on the belly. .Mus rattus (p. 365). 



aa. Size small. Length less than 200 mm Mu£ musculus (p. 366). 



u 6 



d 

 Fig. 59.— Mus musculus. a. Forefoot; 6, Hindfoot; c, Eab; d. Tail. 



MUS NORVEGICUS Erxleben. 

 HORWAY RAT; BROWN RAT; WHARF RAT. 



[Mms] norvegieus Erxleben, Syst. Regni Anim., 1, 1777,p. 318 (original description). — 



Elliot, Field Col. Mus.,Zool.Ser., IV, 1904, p. 164 (Mam. Mid. Am.). 

 M-m norvegieus, Rehn, Proc. Biol. Soc. Washington, XIII, p. 167, Oct. 31, 1900. — 



Miller and Rehn, Proc. Bost, Soo. Nat. Hist., XXX, No. 1, Dec. 27, 1901, p. 



65 (Syst. Results Study N. Am. Mam. to close of 1900). 

 Mus deeumdnus Palias, Glires, 1778, p. 91, No. 40. 

 [Mus\ decumanus, Elliot, Field Col. Mus., Zool. Ser., II, 1901, p. 117 (Synop. Mam. 



N. Am.). 



Type-locality. — Norway. 



GeograpJiical range. — Cities and larger settlements. 



Description. — Normal coloration grayish-brown above, dirty gray- 

 ish-white below; upper surface more or less Uned with black hairs, 

 especially in the vertebral area, which is also grayest. Feet soiled 

 grayish-white. Tail dusky above and whitish below, though not dis- 

 tinctly bicolored. Tail usually shorter than head and body, and so 

 sparsely short-haired as to show the annuli plainly. 



Remarlcs. — This cosmopolitan species is found sparingly in the 

 cities of the Southwest. In Arizona and New Mexico the white 

 domestic variety is most common, though brown ones were occa- 



