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Show 466 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, this tooth (fig. 27, i4, i5, ia) shows indications of at least three and is obviously a fusion of these teeth. It seems to m e probable that the partial calcification of this missing incisor might cause the fusion of these two teeth, which are only separated by a slight 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 well shown in Bidelphys. 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 Bidelphys does not prove necessarily that the missing tooth is the 5th incisor, for we have seen that in the upper jaw of Macropus no diastemata remain to show where the suppressed teeth were situated. Nevertheless, we may provisionally allow that this is the case, and regard the three lower incisors of Petrogcde 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 probabdity this is also the condition in the Rodentia, 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 Esihonyx, then the 3rd have disappeared, and the 1st is smaller than the second, as in Psitta-cotherium or in Ccdamodon, where the 1st has disappeared, which form Cope regards as the ancestor of the Rodentia. From the study of the development of the incisors we have seen that in connection with 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 enumerated 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 Bidelphys, 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 Bidelphys, 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 pm2. In the other forms the dental lamina was invariably present in this region, and often presented irregular swellings but nothing definite. This appears to m e to be strange, as in Petrogale there is a large diastema, even in the young animal, between the |