CHAP. Vl] THE DENTAL SYSTEM OF THE PRIMATES 281 



Before leaving the Cynodontia, two more features must be 

 noticed. 



The dental formula is extraordinarily similar to that postulated 

 by Osborn for the ancestral form of mammalian dentition, viz. 



1 1 C i Pm | M f, 

 with which we may compare 



If C{ Pm-f M}, 

 the probable formula for the Cynodont form Diademodon. 



Again a very early mammalian type (Dryolestes) from strata 

 of the Jurassic period provides a mandible with the formula 

 I ¥ C T Pm 1 M Ft0 g. 

 Lastly, the relation of upper and lower post-canine teeth is 

 a matter of great importance. In the Cynodontia, as in the 



Fig. 202. Dentition of an aboriginal native Australian woman, shewing the 

 alternation of the teeth in the upper and lower jaws. This figure should be com- 

 pared with Figs. 149 and 201. (From Skull No. 2115, Mus. Anat. Cant.) 



Jurassic Dryolestes, the upper and lower teeth alternate. Thus 

 each tooth in the upper jaw comes down behind the posterior 

 border of the lower tooth, between it and the following one. Or, 

 as in Sesamodon, an upper tooth rests partly on each of two of the 

 lower crowns. Such alternation is clearly therefore the ancestral or 

 primitive mode, and as it has been retained in many of the Primates, 

 its significance is very great. In the Hominidae (Fig. 202), such 

 alternation is shewn by Choquet {L'Odontologie, 1908, Jan. Fev.) 

 to exist, but only in a minority of instances (38°/ ). But the early 

 establishment of such alternation in the phylogeny of the mam- 

 malian types is of the first importance, inasmuch as the develop- 

 ment of secondary cusps has been determined thereby to no small 

 extent. This subject will demand closer attention in the next 

 section (VI). 



