1262 THE RODENTS 



dack district, remaining with their parents only a few months. The number in a 

 litter is usually from four to six. When the young first leave their parents they 

 take refuge in stone walls, hollow logs, or even hollow trees habits quite different 

 from those of their Old- World cousins, and showing a marked approximation 

 toward the chipmunks. 



In some parts of New Hampshire, woodchucks are so numerous as to cause seri- 

 ous inconvenience to agriculturists, and rewards have consequently been offered by 

 the State for their destruction. 



Remains of extinct species of susliks occur in the higher Tertiary 

 rocks of Europe; in addition to which the Upper Eocene beds of 

 France yield evidence of an extinct but apparently allied genus, known 

 dents as Plesispermophilus. More primitive are the forms described as Plesi- 



arctomys, which, while showing certain resemblances both to the mar- 

 mots and the squirrels, have triangular three-cusped upper molar teeth, and no 

 bony process defining the hinder limit of the upper border of the socket of the eye. 

 These Rodents are found in the Middle Tertiary deposits both of Europe and North 

 America. 



THE FLYING SQUIRRELS 



Genera Stiuropterus, Pteromys, and Eupetaurus 



The flying squirrels of the Northern Hemisphere, which are divided into the 

 three generic groups named above, are sufficiently characterized by possessing a 

 parachute-like membrane extending from the sides of the body to the toes, and sup- 

 ported in front by a cartilage attached to the outer side of the wrist. There is also 

 another membrane connecting the sides of the neck with the fore-limb, and there 

 may be likewise one between the hind-legs and the root of the tail. The molar teeth 

 are of a very complex type, and as their skulls differ considerably from those of all 

 the preceding genera, the flying squirrels are now regarded as indicating a distinct 

 subfamily. Moreover, as the group is known to be of considerable antiquity, it is 

 not improbable that it has no generic affinity with any of the foregoing genera but 

 that it traces its descent to some totally extinct group of the family. With the ex- 

 ception of one North-American species, and a second from Siberia and Eastern 

 Europe, the flying squirrels are confined to the Indian and Malayan regions. They 

 are all exclusively nocturnal in their habits, and mainly inhabit forest regions, al- 

 though one may frequent rocks alone. Their mode, of flight and general habits ap- 

 pear to be very similar to those of the flying lemur, and when leaping from tree to 

 tree they utter sharp piercing cries which are familiar to all who have traveled 

 in regions where flying squirrels are to be found. 



The lesser flying squirrels, constituting the genus Sciuropterus, are 



\ those which have the lowest crowned molar teeth in the group, al- 

 Squirrels 



though there is considerable variation in the structure of these teeth. 

 All the members of the genus are distinguished by the parachute-like membrane 

 along the sides being of moderate width, and by the rudimentary condition of the 



