144 



THE GNAWERS OR RODENTS. 



locusts, overwhelm the land and leave de- 

 struction and desolation behind them. Their 

 enemies also multiply rapidly indeed, but are 

 helpless in face of such prodigious swarms, 

 which always leave enough to continue their 

 species. 



THE SQUIRREL FAMILY 



(SCIURIDA). 



The family of the squirrels comprises a 

 pretty large number of forms, which vary 



between the two extremes presented by our 

 ordinary squirrels, the agile climbers, and the 

 sluggish clumsy marmots, which live almost 

 entirely underground. But the intermediate 

 forms are so numerous that it is impossible 

 to break up this family into any subdivisions. 

 It is characterized as a whole by the structure 

 of the teeth, the skeleton, and the feet. In 

 the upper jaw there are usually five, in the 

 lower, four cheek-teeth, with triple or quad- 

 ruple roots and triangular crowns; the internal 

 heel and the division of the tooth into external 



Fig. 202. — The I'aguan or Brown Flyiiig-S(iuirrel {Pteroniys pcttiiinsta). 



points present the >--form so characteristic 



of the insect-eaters. In the majority these 

 cheek-teeth retain more or less sharp jaeaks 

 and tubercles even when worn, but others 

 have the grinding surface flattened by use so 

 as to present more or less complicated folds. 

 The first upper cheek-tooth is small and in 

 some species is soon shed. In the skull we 

 are struck by the longish nasals, which 

 become broad at the end so as to support 

 the blunt snout. The frontal bone carries a 

 considerable process, which forms in the rear 



