46 MAMMALIAN DENTITION 



type also appeared for the first time. In consequence of the 

 development of the coronoid process and the greatly increased 

 size of the dentary element in which the teeth are lodged the 

 Therocephalian mandible began to assume a somewhat mam- 

 malian appearance. 



The Cynodonts in general were much more mammal-like 

 than the Therocephalians. They possessed a secondary palate, 

 a large median unpaired vomer and two occipital condyles. 

 The dentary increased so greatly that it constituted almost the 

 entire mandible. The teeth differentiated into incisors, canines, 

 premolars and molars. According to Broom it is probable that 

 these mandibular and dental changes together with a narrowing 

 and deepening of the snout resulted in the formation of the 

 secondary palate. The method of articulation of the mandible 

 with the skull even in Cynodonts is however still typically 

 reptilian. 



If then Dr. Broom's theory proves correct Ave may take it 

 that in Triassic times the mammalian ancestors were either 

 Cynodonts or Cynodont-like Therocephalians. One feature dis- 

 tinguishing these creatures from other more primitive Reptiles 

 was the possession of Avell-developed limbs by which the animal 

 was raised off the ground. Our common Horned Toad has 

 limbs developed to the stage of supporting the body above the 

 ground only in the act of walking. It can represent for us the 

 early Reptile flattened dorso-ventrally, the limbs of which be- 

 sides subserving the more primitive function of locomotion are 

 becoming adapted also for the purpose of support. It has been 

 suggested that the similar change of gait exhibited by the 

 Therocephalians was due to development of the brain but it 

 is altogether inconceivable that at so early a stage of animal 

 history the brain could have such influence upon the body. It 

 is more natural on the contrary to believe that conversely 

 changes in the brain resulted from the altered mode of locomo- 

 tion. The Cynodonts show a relatively great development of 

 the cerebellum which seems to be accounted for by the increased 

 coordination of movement entailed by the new walking habit 



