n6 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



Therefore, while it is undeniable that some cases of supernumerary 

 teeth are teratological with a basis that may be largely physiological and 

 mechanical, it cannot be admitted that there are no cases of which 

 phylogenetic influence is the chief determining factor. The objectionable 

 term reversion need not be applied to them but it might be justified to 

 refer to them as examples of persistence of ancestral characters. Aside 

 from what has been said above, it is evident to anyone familiar with the 

 dentition of many species of mammals, that cases are innumerable in 

 which a nonfunctional or slightly functional tooth is present in one 

 species and normally absent but occasionally present in a closely related 

 species. Among bats and rodents instances are particularly abundant. 

 The homology of such a tooth is invariably too clear to be questioned 

 for a moment. 



That the fifth lower unicuspid sometimes found in Ccenolestes is 

 evidence of an ancestor in which this tooth was normally developed may 

 not now be proved conclusively but the evidence from paleontology, 

 from embryology, and from phylogenetic series is sufficient to render it 

 exceedingly probable. That the primitive diprotodont incisor formula 

 was at least is also strongly indicated. Hence the main points in the 

 present connection seem fairly certain, that is, that primitive diproto- 

 donts had numerous teeth and that the diprotodont modification may 

 have arisen prior to any reduction from the primitive number. 



UPPER INCISORS. 



Description. The median incisors are set near the end of the pre- 

 maxillae in nearly terminal position, only a slight shelf of bone extending 

 in front of them. Their alveoli are more elevated than those of the suc- 

 ceeding teeth and they are separated from the next incisors by a slight 

 space. They are inclined forward slightly away from the perpendicular 

 and their points exceed the lateral incisors. They are separated at the 

 base sufficiently to permit the insertion of a needle or bristle but their 

 points are closely in contact. Their cutting edges are beveled and receive 

 the extreme tips of the lower incisors. They are closely similar in shape 

 to corresponding teeth in the Australian diprotodonts, especially some of 

 the smaller macropods. They have relatively broad faces which suggest 

 the undifferentiated median incisors of Perameles quite as much as they 

 do those of the didelphids. The median upper incisors of typical poly- 

 protodonts (dasyures and didelphids) are usually slender and peglike 

 and in most cases the tips are not in contact although occasional speci- 

 mens may be found having some resemblance to those of C&nolestes. 



The three succeeding incisors (second, third, and fourth) are wholly 

 lateral in position and the series on one side is almost exactly parallel 



