THE BROWN OR COMMON RAT 631 
the young commence to leave the nest and eat—they are very 
lively. At the twenty-third day their white coat begins to 
attain the yellowish tints of the adults ; they incessantly run, 
jump, and climb. By the twenty-sixth day, and sometimes 
even by the twenty-first or twenty-second day, they can leave 
their mother, but if with her they may suckle to the twenty- 
eighth day. The young have extraordinary vitality: one— 
only two days old—lived after forty-eight hours’ exposure in 
September to the external air on a metal plate (Lataste, 
344 and 374). 
Rats have a considerable development of voice, frequently 
squeaking loudly; Johnston (240) describes one as uttering 
a thin metallic ‘‘skikking”” sound when angry, or a grunting, 
murmuring noise when amorous. 
Genus MUS. 
1758. Mus, C. Linnzeus, Syst. Mat., roth ed., i., 59; genotype, by tautonymy, Mus 
musculus (G. S. Miller, Proc. Biol. Soc., Washington, xxiii., 19th April 1911, 59) ; 
in part of most authors. 
1814. MUSCULUS, Rafinesque-Schmaltz, Précis des Découv. et Travaux Somiologiques, 
13; a substitute for Mus. 
1837. LEGGADA, J. E. Gray, Charlesworth’s Mag. Nat. Fist., i.. 586; genus for 
L. booduga, Gray. 
1842. MicRoMyYs, Lesson, Wouveau Tableau, Mamm., 139. 
1845. DRyMOMYS, Tschudi, Fauna Peruana, 178; based on D. parvulus, Tschudi= 
Mus musculus, Linnzus (see Thomas in Palmer, /udex Gen. Mamm., 246, 1904). 
1876. NANNOMYS, W. Peters, Monatsber. k. preuss. Akad. Wiss., Berlin, 480; based 
on J. seculosus, Peters. 
1881. Acromys, E. L. Trouessart, Bull. Soc. @Etudes Sci. @ Angers, X.. 1333 a 
synonym of Drymomys, Tschudi. 
1896. PSEUDOCONOMYS, Rhoads, Proc. Acad. Nat. Sci., Philadelphia, 531; sub- 
genus based on Mus (Pseudoconomys) proconodon, Rhoads, from Western Somali- 
land. 
1900. DRyoMys, Philippi, Az. Mus. Nac. de Chile, xiv., 20; a modification of 
Drymomys, Tschudi. 
As now defined, the genus J7/us is restricted to the House 
Mice and their allies; these, according to Miller, comprise 
about twenty-five distinct forms, of which seven are represented 
in western Europe. 
This is a perfectly natural group enjoying a wide natural 
