MUSEUM OF COMPARATIVE ZOOLOGY. 227 



59. Mus rattUS Linn. Black Rat. Abundant in the farming 

 districts, but rare wherever the brown rat is numerous. In the vicinity 

 of Boston and of the larger cities generally it seems to be quite un- 

 known. Twenty or thirty miles from the coast, and at a little distance 

 from the large towns along the railways, it becomes numerous, and the 

 only species there found. The brown rat is its mortal enemy. With 

 age this species changes from black to gray, very old individuals becom- 

 ing very light colored. 



60. Mus musculus Linn. House Mouse. Everywhere a 

 numerous pest. Is frequent in the fields under stacks of grain as well 

 as in houses and outbuildings. 



61. Hesperomys leucopus LeConte. {H. leucopus and H. 

 myoides Baird.) White-footed Mouse. Deer Mouse. A com- 

 mon species of the fields and woods. In winter it (sometimes at least) 

 retires to a warm nest in a hollow stump or log, in which in severe 

 weather I have found five or six together in a torpid state. 



No species of our Muridce, excepting possibly the Jaculus hudsonius, 

 presents so great variations in color with season and age as the present. 

 The young for the first two or three months, or till nearly full-grown, are 

 dark slate or plumbeous above, somewhat lighter below. From the casting 

 of the winter coat in spring till late in autumn the adult differs more or 

 less in color with almost every individual, none presenting the bright yel- 

 lowish or ferruginous brown seen in winter and early spring, but every 

 stage between it and the plumbeous hue of the young ; the adult being 

 also more or less dusky for some time after moulting. Generally there is 

 a darker band along the back, varying in width in different specimens, 

 and in distinctness of outline ; sometimes, however, the back is uniform in 

 color with the sides. The variation in size is also considerable between 

 specimens apparently fully adult. The tubercles on the soles of the hind 

 feet, on which specific distinctions are sometimes based, vary both in rela- 

 tive size and position. The posterior one is usually situated midway be- 

 tween the toes and the heel, but sometimes more posteriorly or more an- 

 teriorly. The next one is placed between this and the third, and is usually 

 nearer to this than to the first, it being sometimes opposite to the third. 

 The third anterior tubercle occasionally has a minute supplemental one at 

 its outer base. But the most variable character consists in the relative 

 lencth and number of the caudal vertebrae. About one fifth of the Massa- 



