100 Mr. Brookes on a new Genus of the Order Rodentia. . 



may be noticed in the squirrel and Pteromt/s. The toes are 

 three, the middle one being the longest and the inner one the 

 shortest. 



From this the hinder extremity of Dipus differs most essen- 

 tially. Its femur is arched, with the convexity forwards, and 

 is only half the length of the tibia. The fibula is short, and 

 extends to just below the middle of the tibia, where these 

 bones are firmly ossified together : the metatarsal bone, which 

 is about two-thirds of the length of the tibia, is single, and is 

 terminated by three nearly equal toes, the lower part of the limb 

 bearing in the skeleton a striking resemblance to that of a small 

 tridactylous wader. 



To this outline of the more remarkable particulars exhibited 

 by the skeleton of Lagostomus, and of the numerous and im- 

 portant differences which exist between it and that of the Jer- 

 boa, a few observations may be added respecting its relation 

 with those of other rodent (Quadrupeds nearly approaching to it 

 in size. 



The Lagostomus has 12 ribs, and consequently twelve dorsal 



— [vertebrae. 



Jerboa 



Squirrel 



Rabbit 



Marmot 



Coypus 



Agouti 



Urson . 



Capromys 



The Lagostomus 

 Jerboa 

 Squirrel 



12 

 12 

 12 

 J3 

 13 

 13 

 14 

 16 



7 

 7 

 7 



lumbar vertebrae. 



The 



