48 MAMMALIAN DENTITION 



After this brief general outline it will be instructive to 

 turn to some of the Cynodonts most nearly related to Mammals 

 in order to observe the gradual appearance of mammalian 

 traits. 



The two South African Triassic Cynodonts Bauria and 

 Sesamodon are probably nearest to the direct line of mam- 

 malian evolution. Although almost certainly a Cynodont Bau- 

 ria retains many of the Therocephalian features and thus rep- 

 resents a comparatively early ancestral stage. In spite of the 

 fact that it is less mammal-like than certain other Cynodonts 

 Bauria exhibits some mammalian characters which the higher 

 forms have lost. Sesamodon is interesting because it displays most 

 of the mammalian characters of Bauria and in addition presents 

 the majority of those found in other higher Cynodonts. The 

 relations of the various mammal-like Reptiles, now to be dis- 

 cussed, to each other and to the hypothetical mammalian an- 

 cestor are diagrammatically shown in Broom's table (Fig. 9). 



Examination of the restored skull of Bauria (Fig. 10) shows 

 that the teeth fall into incisor, canine and premolar-molar series. 

 The last mentioned are numerous, comparatively small, and 

 simple in form ; the canine as in Mammals is represented by the 

 first maxillary tooth. The dentary element forms the greater 

 part of the mandible Avhich exhibits a well marked coronoid 

 process but articulates with the skull in typical reptilian fash- 

 ion by a joint between the articular element and the quadrate, 

 the latter invisible in the illustration because of the laterally 

 overlapping squamous. The skull no less than the mandible 

 and teeth presents a general mammal-like appearance. The 

 temporal fossa is large and incompletely separated from the 

 orbit by a process of bone which though analogous is not 

 homologous with the process separating the temporal fossa from 

 the orbit in Mammals. In the Cynodont this process is formed 

 by a special bone, the post orbital, Avhich disappears in the 

 Mammal and is replaced by the postorbital process of the fron- 

 tal. The zygomatic arch is not powerful and no process arises 

 from the zygomatic bone to meet the postorbital. Strikingly 



