CHAP. Vl] THE DENTAL SYSTEM OF THE PRIMATES 



279 



Other palaeontologists objected to this exclusion of the heterodont 

 reptiles from the ancestral history of the mammals. They pointed 

 to the remarkable characters of some extinct reptiles known as the 

 Theriodontia 1 , and referable to the Triassic period. The last ten 

 years have seen great accessions to our knowledge of those animals 

 and it is almost impossible to resist the conclusion that the 

 mammals passed through an ancestral stage corresponding to that 

 of the Cynodontia (a particular sub-order of those fossil reptiles). 

 Detailed evidence is accessible in the exhaustive memoirs pub- 

 lished by Dr Broom 2 . 



A representation of the skull of one of the Cynodontia, viz. 

 Sesamodon browni 3 , is shewn in Fig. 201. The skull of this Triassic 



Fig. 201. Skull of Sesamodon browni, a Triassic reptile having a dentition 

 similar to that of a mammal. (After Broom.) 



reptile exhibits clearly the chief point of interest in the present 

 connection, viz. that the reptilian dentition was already heterodont, 

 to the extent that incisor, canine and post-canine types of tooth 

 can be easily distinguished. In some of the Cynodontia, a dis- 

 tinction can be drawn between the premolar and the molar series 

 also 4 . 



1 Cf. Woodward, Vertebrate Palaeontology, p. 247. 



2 Not the least striking result is the demonstration (Sept. 1913) of a deciduous 

 dentition (in a Cynodont skull) resembling that of Mammals. 



3 Cf. Broom, P.Z.S. 1911, p. 893. 



4 The Cynodontia also serve to discountenance a view of the ancestry of 

 mammals which at one time gained some support. That view excluded a reptilian 

 stage, and carried back the mammalian line directly to an "Amphibian " stage to 



