ARVICOI.A RIPARTUS. I 75 



operation of the same cause, come to be surrounded by slowly-grow- 

 ing dome-shaped chambers. These increase in size until the spring 

 thaws, in March and April, melt away their roofs, thus admitting the 

 light and cold. They are then deserted. During snow-shoe tramps 

 over the fields at this season I have often noticed holes, from a few 

 inches to a foot in diameter, appearing as if sharply cut in the surface. 

 On inspection, they invariably proved to be the summits of these 

 dome-shaped cavities, and a nest was always found at the bottom of 

 each, surrounded by a zone of bare ground. They ranged from one 

 to two feet (approximately 300 to 600 mm.) in diameter, and most of 

 them were two feet in height. From the bottom of each chamber 

 numerous runways and burrows penetrated the snow in all directions. 

 Some followed along directly upon the ground, while others sloped 

 upward at various angles. Many ran horizontally at varying levels, 

 resting upon the dense strata that indicated the surface lines at 

 different times during the winter. Near each nest was one or more 

 burrows that reached the surface and contained considerable accumu- 

 lations of the animal's dejections. These seemed to be watch holes 

 where the Mice came regularly to look at the prospect outside. 



Meadow Mice sometimes, but not often, travel upon the snow, and 

 they occasionally stray so far that they are unable to find the holes 

 through which they came up. If this happens when there is a hard 

 crust, tlirough which they cannot burrow, they wander aimlessly about 

 for a while and finally perish from the cold. In March and April I 

 have several times found them frozen to death upon the crust. 



They are always present in greater or less numbers, but are 

 not often sufficiently abundant to direct the attention of the farmer 

 to their depredations. Occasionally, however, they multiply to such 

 an alarming extent that the most superficial observer is impressed 

 with the magnitude of their ravages. The)' devastate the meadows, 

 grain-fields, and orchards of the farmer, and ruin the nurseries of the 

 horticulturist. Whether these periodical invasions are the result of 

 unchecked reproduction, or of migration, has not been positively 



