466 MB, M. F. WOODWARD ON MAMMALIAN DEKTITION. [May 2, 



this tooth (fig. 27, i\ ^^ i") shows indications o£ at least three fangs, 

 and is obviously a fusion of these teeth. It seems to me probable 

 that the partial calcification of this missing incisor might cause the 

 fusion of these two teeth, which are only separated by a sUght 

 interval in the foetus. 



The comparatively large size of the vestigial 5th incisor in the 

 Macropodidse is obviously accounted for by the late development of 

 the 3rd adult incisor, whereas the 2nd and 3rd, which are functional 

 teeth in the Polyprotodonts, have been dwarfed by the early 

 development and large size of the 1st and 4th incisors. The 

 commencing enlargement of the 1st incisor is v^eW shown in 

 Didel2}^iijs. 



With regard to the lower incisors, evidence is wanting to show 

 which of the Polyprotodont's teeth these represent. The close 

 approximating of the four lower incisors of Didelphys does not prove 

 necessarily that the missing tooth is the 5th incisor, for we bave 

 seen that in the upper jaw of Macrojuis no diastemata remain to 

 show where the suppressed teeth were situated. Nevertheless, 

 we may pronsionally allow that this is the case, and regard the 

 three lower incisors of Petvogalc as representing the 1st, 2nd, and 

 3rd, the 1st and 3rd being vestigial. The great functional lower 

 incisors of the Macropodidse are therefore the over-developed 2nd 

 incisors. 



Cope (2) has shown that in all probability this is also the 

 condition in the Eodentia, there being strong evidence to believe 

 that the single pair of large lower incisors are the 2nd ; the 1st and 

 3rd have first become reduced as in Esthonyx, then the 3rd have 

 disappeared, and the 1st is smaller than the second, as in Psitta- 

 cotlierium or in Calamodon, where the 1st has disappeared, which 

 form Cope regards as the ancestor of the Eodentia. 



From the study of the development of the incisors we have seen 

 that in connection A^ith the 1st, 2nd, 4th, and 6th above, and the 

 1st and 2nd below, rudimentary successional teeth ("Ersatzzahne") 

 are to be found at one stage, thus proving that the teeth enu- 

 merated above are present in some form or other in both 

 dentitions, and that the three incisors above and the one below in 

 the adult, belong to the 1st, or milk dentition. This is in perfect 

 accord with Kiikenthal's (5) observations on the incisors of 

 Didelphys, all of which he shows to belong to the 1st dentition. 

 In no case have I been able to determine as to which dentition 

 the canine is to be referred. In Didelphys, however, Kiikenthal 

 saw something which he considered to represent a rudimentary 

 successional tooth, but it was evidently, from his description, very 

 slight. 



Only in Macropus giganteus have I been able to find any certain 

 trace of the missing premolars ; in this case the tooth found was 

 probably ^:)«ij. In the other forms the dental lamina was invariably 

 present in this region, and often presented irregular swellings but 

 nothing definite. This appears to me to be strange, as in Petrogale 

 there is a large diastema, even in the young animal, between the 



