192 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XL 



posed of soft grass, etc.; it is spherical, and the small internal cavity 

 is entered through a narrow opening on one side. In cultivated fields 

 the burrows are frequently dug at the roots of fruit-trees, the bark of 

 which is often gnawed, sometimes causing great injury. In nurseries, 

 fruit-trees are often taken up and 'heeled in'; that is, laid down close 

 together, with the roots placed in a trench, and then covered in such 

 manner that they are kept safely in a very small space, and can be 

 readily pulled out when desired. The loose earth among the roots 

 of these offers an inviting habitation to the mice; and, in nurseries in- 

 fested by them, they will be found burrowing in almost every lot of 

 trees thus buried, where they feed upon the bark of the roots, and thus 

 cause serious damage. In the fall, they are often found in corn-shocks, 

 making a nest among the stalks, though they do not so often burrow 

 under these as the arvicolas. But, during winter, they may be tracked 

 in corn-fields from their burrows to the neighboring corn-shocks, which 

 they have visited for food. In spring, the young are always produced 

 in burrows. During the summer, however, they occasionally are 

 observed in nests, under bits of wood or bunches of hay, on the surface 

 of the ground. In autumn, I have found nests of the young in small 

 burrows only a few inches below the surface, or under an inverted sod. 

 I have never observed more than one pair of adults occupying the same 

 burrow; and, unlike the Mus leucopus, this species never appears to 

 be gregarious. 



"This mouse must be very prolific. I have found the young in 

 March and April, and observed two females, each with five young, 

 apparently but a few days old, about the tenth of November, while 

 they are found in every intervening month. In nearly every instance 

 within my observation the number of young produced at a birth has 

 been five. I once found six, and have at times, though rarely, seen 

 three or four. The young are found attached to the teats, as in the 

 species last described; and a female was seen to carry five for several 

 rods in this way, jumping along rapidly despite their weight. As 

 soon as they are able to take care of themselves, the young leave the 

 mother. In summer, I have several times found one apparently but 

 a few weeks old, living alone in a nest made by himself. In spring, I 

 have always found the old male living with the female and young; but 

 during the summer, I have sometimes observed the male leading a 

 solitary life, and the females and young in burrows by themselves. 

 The food of this mouse, on the prairies, appears to be herbaceous plants, 

 with their seeds; but I have been unable to see that it ever digs for 

 roots. It is interesting to observe that this, like the Mus leucopus, 

 seeks its food on the top of the ground, running on the snow in winter 



