ANCESTRY OF THE MAMMALIA 59 



foregoing group, occupies the buccal aspect of the tooth. The 

 paraconid unlike the condition in the Triconodont is not sub- 

 equal with, but smaller than, the metaeonid which lies internal 

 rather than postero-internal to the protoconid. There is also 

 a further structure, a kind of heel, developed on the posterior 

 aspect of the tooth; this has received the name talonid. In 

 Amphitherium there is but one cusp, the entoconid* upon the 

 heel. No cingulum is developed. The lower molars of Amphi- 

 therium differ very considerably from those of Triconodonts 

 in the relationship in size between the paraconid and metaeonid, 

 in the position of the metaeonid relative to the protoconid and 

 in the absence of the internal cingulum. The mandible also of 

 Amphitherium is built upon a plan slightly different from that 

 of the Triconodont. The angle is not inflected in one species 

 though it is in another and whereas the condyle seems to be 

 sessile and the coronoid process broad in Amphitherium itself 

 the related form, Peramus, shows a narrowing of the coronoid 

 and a slight suggestion of a neck on the condyle. 



In Amphitherium as in the Triconodonts there is a sharp 

 differentiation between premolars and molars and the former 

 teeth increase in size from before backward until the last is 

 even more elevated than the first molar. In both respects 

 these Jurassic Mammals resemble Diademodon. The tooth 

 formula : 



P-.M-. 

 4 / 



is fairly characteristic of all these three types and may have 

 been a primitive feature inherited by both Triconodonts and 

 Trituberculates from a remote common ancestor but the marked 

 differences between the teeth of the two Jurassic groups of 

 Mammals indicate that they must have evolved on long separate 

 and divergent lines. 



We know the upper jaws of three examples of the Tritu- 

 berculates and to these should probably be added a fourth. As 



*Behind the metaeonid. Later we shall find that two other cusps develop on the 

 heel, the hypoconid behind the protoconid, and the hypoconulid axially and pos- 

 teriorly. 



