ii4 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, VOL. XIV. 



agree with the macropods in the ontogeny of the upper jaw. The dental 

 formula of the primitive diprotodont would then have been as follows: 



6 i 



The earliest known mammals, whether they were pro-marsupials or 

 not, furnish no indication that such a large mammalian incisor formula 

 ever existed and it must be admitted that the evidence in favor of it is 

 little more than suggestive. Still, if early mammals only were con- 

 sidered, it would be almost as difficult to believe in an incisor formula of 

 $ as one of I, for the usual number of lower incisors in the Triconodonta 

 and Trituberculata was four. Both Thomas (1888) and Winge (1895) 

 have presented evidence that the primitive formula was at least f and 

 the case for it must be regarded as considerably stronger than that for 

 one of i so to that extent the testimony of the little known "pro- 

 marsupials " is weakened. In fact, in this as in some other respects, it 

 is evident that the long known Mesozoic mammals are extremely liable 

 to misinterpretation. Evidence is accumulating to show that even 

 these early triconodonts were not altogether generalized and many 

 connectant forms are needed before the main mammalian stem will be 

 definitely revealed. The highly specialized nature of the dentition in 

 some of the theriodont reptiles, including both secodont and crushing 

 types and even showing a mammalian succession, suggest that some 

 basic lines of the general evolution of mammalian teeth may have been 

 laid down in these early reptiles and, although lost in the triconodonts, 

 persisted in still later forms the immediate ancestors of which are 

 unknown. Although most of these theriodonts had only four upper 

 incisors on a side, there were some (e. g. Pristerognathus) which had as 

 many as six. Hence a mammal with six is theoretically open to no serious 

 objection. In any case it is clear that as early as Miocene times caeno- 

 lestids were at least as primitive as polyprotodonts in the number of the 

 teeth in the lower jaw. The number in the upper jaw is one less than in 

 the didelphids and certain of the peramelids, which have five upper 

 incisors, most other polyprotodonts having four. 



The lateral lower incisors of Ccenolestes are small and while not wholly 

 functionless are apparently approaching that condition, so variations 

 in their number are more to be expected than would be the case with 

 fully functional teeth. It is possible that the occasional appearance of 

 an extra unicuspid has no reference to ancestral conditions and therefore 

 that all the specimens of the extinct forms Halmarhiphus and Garzonia 

 are abnormal. Evidently this would be the conclusion of Bateson (1894) 

 who places no confidence in "reversion" or ancestral influences as the 



