RODENTIA. 



713 



It is possible that in their transition from terrestrial to marine life, 

 the Cetaceans may have passed through a stage in which they lived in 

 fresh water. 



Fig. 316. — Vertebra, rib, and sternum of Balcenoptera. — 



From specimen in Anatomical Museum, Edinburgh. 



C, Centrum ; n.a., neural arch ; n.sfi., neural spine ; /./., transverse 



process ; R. t rib ; St., sternum. 



Order Rodentia. 



Rodents are represented in all parts of the world, and by 

 more species than any other order of Mammals. Most of 

 them are small and terrestrial. They are typically vegetarian, 

 and gnaw their food in a characteristic way. 



The dentition is characteristic. The incisors are chisel- 

 edged, for, as the enamel is either restricted to the front or 

 is at most thin posteriorly, the back part wears away more 

 rapidly. The incisors are rootless, growing from persistent 

 pulps, and the same is sometimes true of the back teeth. 

 There is never more than a pair of lower incisors, and in 

 most cases the upper jaw has only a pair. There are no 

 canines, and the skin projects as a hairy pad into the mouth 

 through the gap between incisors and premolars. 



The feet are plantigrade or semi-plantigrade, generally 

 with five clawed or slightly hoofed digits. Clavicles, though 

 often rudimentary, are generally present. The scapula has 



