ARVICOLA RIPARIUS. 
175 
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, through 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. They 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 
