116 MICROSCOPIC ANATOMY OF THE TEETH 



A rodent type of dentition is found in two animals widely 

 separated from the Rodents and from each other. In 

 Australia the marsupial Wombat has long scalprif orm teeth 

 growing from persistent pulps, and in Madagascar a similar 

 dentition is found in a Lemur, the Aye-aye (Cheiromys) . In 

 both of these countries indigenous true Rodents are un- 

 known. As C. S. Tomes says (18 c), 'In past times ... a large 

 number of animals, widely apart from one another in their 

 affinities, have more or less approximated to a rodent 



FIG. 67. Enamel of Rat showing fibrous nature. ( x 450. ) 



dentition, and as this adaptive resemblance would give to 

 them advantages in getting access to food which was denied 

 to animals with relatively blunt and weak front teeth, it is 

 possible that some of these may have left descendants, and 

 that hence the existing Rodents may have sprung from 

 more than one stock '. 



Tubes from the dentine passing into the enamel are seen 

 in abundance in the teeth of the Rodent Jerboa (Dipus), 

 and Von Ebner says it is not difficult to find tubes in the 

 enamel of other Rodents, as mouse and guinea-pig, but these 

 are not continuous with the dentinal tubes. He considers 

 that a resorption of the first -formed dentine takes place 

 whereby the ends of the dentinal tubes are left open, and 



