THE HEAD AND NECK. 127 



primitive condition of dentition there appears to 

 have been no such interval, all the teeth being in 

 contact j and this condition is retained, or perhaps 

 regained, by man, almost alone among existing 

 mammals. Already, in Phenacodus, there was an 

 indication of this diastema, and throughout the 

 whole series of Perissodactyles which lead up to the 

 Equidce there has been a gradual increase of its 

 length. 



Molar Teeth. The cheek-teeth, or molars, exclud- 

 ing the rudimentary and inconstant anterior pre- 

 molar, spoken of before, are six in number, above 

 and below, on each side (see Fig. 16, pm 2 to m z ). 

 They are all in such close contact, by broad surfaces 

 fitting tightly against each other, that they form to- 

 gether one solid mass, presenting a grinding-surface 

 composed of substances of various densities, and 

 therefore projecting at slightly different levels, inter- 

 woven in such an intricate pattern as to form one of 

 the most efficient natural millstones imaginable. 



A distinction must be pointed out among these 

 teeth. In a great many animals their form differs 

 so much that they are readily separated into an 

 anterior set of simple character, and a posterior set, 

 larger, broader, and with more complex crowns and 

 roots j and when it was discovered that these also 

 presented a constant difference in their mode of 



