1910.] The Protodoiita: Drnmalhcrium (Did M irroconodon. 163 



The Protodonta. 



In his description of the lower jaws of the American Triassic genera 

 Droniafherium and Microconodoii, Professor Osborn (1907, p. 19) says, 

 "The chief reason for considering these jaws mamniahan is that they are 

 composed of a single bone, there being no evidence of the separation into 

 dentary, articnlar and angular elements, as in the jaws of reptiles." 



This reason would doubtless be conclusive if the jaws of Cynodont reptiles 

 were not known; but comparison (Fig. 6) of the jaws of Droniafherium 

 (No. 4) and M irroconodon (No. 5) with those of Triholodon (No. 2) and 

 Diademodon (No. 3) reveals the possibility that small angular and articular 

 bones may in fact have been present in life but not preserved in the fossil 

 remains of Dromatherinm and Microconodon already discovered. The 

 condylar process of the dentary in Dromatherium, so far as the cast of the 

 fossil shows, is prolonged backward more than in Diademodon, and may 

 well have touched the glenoid region of the squamosal; but its apparently 

 slender character may indicate that the reduced quadrate-articular angular 

 chain .still retained part of its old supporting fu7iction (c/. pp. 137, 124). The 

 broad corono condylar portion is similar in type to that of the Cynodont 

 genera Cynognathus (Fig. 2, B), Trirachodon, Triholodon and Diademodon. 

 It has also the same gentle forward and downward sweep. 



In Microconodon the incipient development of the angular process of the 

 dentary is similar in position to, but broader than, that in the reptile Diade- 

 modon, while the molars are more advanced in form than, but of the same 

 general type, as those in the reptiles Tribolodon and Cijnognatlnis. Again, 

 the styliform premolars of Microconodon are rather suggestive of those in 

 Diademodon (Broom, 1905.4). 



So far as the evidence shows, the Protodonta differ from and are more 

 primitive than all known mammals, first in the low development of the 

 corono-condylar notch, or sinus, separating the coronoid process from the 

 condyle posteriorly, secondly in the arrangement of the incisors (in Dro- 

 matherium), which extend down on the front of the symphysis. On the 

 other hand, the Protodonta are more progressive than the rej)tiles in the 

 division of the fangs of the molars and in the slightly greater backward 

 prolongation of the condylar process of the dentary. They seem to fore- 

 shadow in several features the coronoid, mandible and cheek teeth of the 

 genus Priacodon Marsh from the Como Beds (Upper Jurassic) of Wyoming. 



There seems accordingly to be considerable reason for accepting Seeley's 

 view (1895.4, p. 90) that the Protodonta may be related to the Cynodontia; 

 but in view of their more progressive characters as compared to the Reptiles, 



