32 



SKETCH OF THE HISTORY OF MAMMALIA. 



their own distinct metatarsal bone. The incisors of the 

 Alactagas are simple, whilst those in the upper jaw of 

 the Jerboas are divided longitudinally by a furrow. 

 The molars of the latter genus are complicated in form, 

 and but little resemble those of the former. They are 

 four in number in the upper jaw, and three in the 



17.— Skull and Teeth of Dipus hirtipes. 



lower; but the first in the upper is a small rudi- 

 mentary tooth, which probably disappears in aged indi- 

 viduals. After a detailed account of the structure of 

 the ginding teeth, M. Cuvier observes that the general 

 structure of the head of the Alactagas and Jerboas is 



