21 



The roof nit in general resembles the black rat, except as to color 

 and texture of fur. Above it does not greatly differ in color from 

 the brown rat, but its underparts are usually more yellowish. The 

 fur is commonly shorter and harsher in texture than that of the 

 black rat, but this difference might not always be apparent in speci- 

 mens of the two forms from the same latitude. The average meas- 

 urements of 50 adult specimens of the roof rat in the collections 

 of the Biological Survey are as follows: Total length, 393.3 milli- 

 meters (15.5 inches); tail, 212.8 millimeters (8.4 inches); hind foot, 

 36.2 millimeters (1.4 inches). 



Both albinism and melanism are frequent among rats, and pied 

 forms also are common. It has been claimed that all the white rats 

 (albinos) of the bird stores are Mus rattus, but albinism is by no 

 means confined to this species. Doctor Hatai found that all the 

 colonies of white rats maintained at the neurological laboratories of 



a, 



FIG. 3A, FIG. SB. Ears of brown rat and Mack rat, showing relative size. 



Chicago University and the Wistar Institute of Anatomy, Philadel- 

 phia, were of the M. norvegicus species. a The same is true of all 

 the albino rats in the collections of the National Museum and the 

 Biological Survey. These collections contain also several spotted 

 rats (gray and white) and sooty-bhick specimens indistinguishable 

 in color from M. rattus, all being undoubtedly of the M. norvegicus 

 species. 



KEY TO THE SPECIES OF MUS IN AMERICA. 



Size small. Total length of adult less than 200 millimeters Mus musculus 



Si/o largo. Total length of adult exceeding 300 millimeters. 



Ears moderate, when laid forward barely or not reaching eye; tail shorter than 

 (rarely equal to) the length of head and body, darker above than below; 

 color of body normally gray-brown above, white below; hind foot 38-46 milli- 

 meters Mus norvegicus 



Ears larger, when laid forward reaching at least to middle of eye; tail longer 

 than head and body, dusky all around; hind foot 33-37 millimeters. 

 Color grayish-brown above, white or yellowish white below. 



M. rattus alexandrinus 

 Color blue-black above, slaty below M. rattus 



I'.iological bulletin, vol. 1.2, pp. 2(>(i-27U, March, 1907. 



