SOCIAL BACKBONED ANIMALS 135 
The Common Prairie-Marmot (Cynomys Ludovicianus), more 
familiarly known as the Prairie-‘‘ Dog”, on account of the barking 
sound to which it gives utterance when alarmed, is a social 
burrowing rodent native to the dry open plains on the east of 
the Rocky Mountain highland. Several individuals live together 
in the same burrow, and a very large number of these homes 
may be associated together into a “city”. The excavated earth 
is partly heaped up into a mound at the entrance to the burrow, 
and this may almost be called a watch-tower, since it is commonly 
occupied by a sentinel, who performs the usual duties of his office. 
On hearing his warning bark the neighbouring individuals scuttle 
down into their underground dwellings. The opening of each 
burrow is somewhat funnel-shaped, some of the earth dug out 
having been regularly arranged round it, and pressed together 
till firm: the use probably being to prevent flooding out during 
sudden showers. Owing to the fact that Burrowing Owls and 
Rattlesnakes are not infrequently found in the dwellings of this 
rodent, the conclusion has somewhat hastily been drawn that the 
three kinds of animal constitute a kind of “happy family ”, living 
together on the best of terms. This, however, is not the case, 
for the Owl simply takes possession of a deserted burrow, while 
the Rattlesnake appropriates one at random, ejecting the rightful 
owner if necessary, and thereafter visits his neighbours, levying a 
tax upon their offspring. The Common Prairie-Marmot is about 
a foot in length, exclusive of the short tail, but a larger and a 
smaller species are also indigenous to North America. The 
former is the Mexican Prairie-Marmot (C. Mexicanus), while the 
smaller sort is the Columbian Prairie-Marmot (C. Columébzanus), 
which inhabits higher ground in the western part of North 
America. Their habits are much the same as those of the 
common form. It appears that there is a winter-sleep in the 
case of individuals which live pretty far north. The True 
Marmots (species of Arctomys) are somewhat larger animals allied 
to the preceding, and like them gregarious burrowers. They 
have a wide range through the northern parts of North America 
and Eurasia, living on plains except in the southern part of their 
distributional area, where they are typical mountain-animals. It 
is a familiar fact that they indulge in a long winter-sleep. 
The Beaver (Castor fiber) is a very large social Rodent which 
formerly had a wide extension in Europe and North America, 
