ANCESTRY OF THE MAMMALIA 51 



molars. In the occipital region of Sesamodon we note two 

 condyles as in Mammals instead of the partially subdivided 

 single median condyle reminiscent of Reptiles found in Bauria. 

 The frontals like those of Bauria and unlike those of other 

 Cynodonts enter into the formation of the orbits as they do in 

 Mammals. Unlike the condition in Bauria the separation of 

 orbit and temporal fossa is complete owing to the large size of 

 the post orbital bone, an element non-existent in the Mammal. 



Before leaving this important group it is necessary to refer 

 to certain features illustrated by other and more specialized 

 species. Very significant is the possibility in many Mammals 

 of a lateral movement of the cheek teeth when the jaws are 

 closed and the incisors locked. This primitive triturating move- 

 ment is already foreshadowed in Sesamodon and in Cynogna- 

 thus (see Fig. 9). 



The number as well as the character of the teeth is impor- 

 tant. Aelurosuchus, the cheek teeth of which are not known, 

 possessed on each side five incisors and a canine in the upper 

 jaw, four and a canine in the mandible ; its dental formula can 

 therefore be expressed: 



l|, Cy,P?,M?. 



Diademodon, (Fig. 12) another Cynodont described by See- 

 ley and Broom, possessed four premolars and seven molars ; its 

 formula therefore is: 



I?, C?,P^M^. 



The lower premolars increase in size from before backward 

 and the fourth projects more from the mandible than the first 

 molar. Each displays two cusps, a larger outer and an inner. 

 The molars are much bigger teeth than the premolars but never- 

 theless exhibit the reptilian character of a single root. Those 

 of the upper jaw are larger; they are broader transversely, 

 compressed antero-posteriorly and between them the lower 

 molars occlude. Each of the mandibular molars like the pre- 



