174 COMPAEATIVE DENTAL ANATOMY 



have similar cusps to the former, when erupted, 

 but soon wear down to the characteristic plates, 

 the separate denticles taking the form of dentinal 

 plates. The grinding face of the molar of the 

 Mastodon had transverse ridges which supported 

 two teat-like tubercles. The first molar had 

 two such ridges and four tubercles. The sec- 

 ond had three bifid, transverse eminences and a 

 tuberculate ridge on the rear of the crown. The 

 third molar had four ridges and occasionally five 

 and a posterior talon. The intervening valleys 

 were at first filled with cementum, but this wore 

 out with use. The extinct Dinotherium had the 

 upper tusks absent and the lower prolonged into 

 downward-curving tusks. It was aquatic in its 

 habits, and employed the anomalous tusks for dig- 

 ging aquatic plants. Its molars were similar to 

 those of the Tapir. 



The Primates (or Quadrumana) comprise those 

 mammals having four hands, — the Lemurs, Mon- 

 keys, and Apes. They are distinguished from all 

 other mammals by having opposable thumbs on 

 both hands and feet which enable them to grasp 

 objects. The Primates include also those mem- 

 bers of the animal kingdom, the Apes, which stand 

 next to man in structure and whose dentition is 

 exactly like that of man. Although thus ap- 

 proaching man, the teeth are still like the lower 

 mammals, arranged in a parallelogram instead of 



