OCR Text |
Show 1893.] MR. M. F. WOODWARD ON MAMMALIAN DENTITION. 467 canine and the third premolar, so that one might reasonably expect to find some trace of these teeth; in M. brachyurus and M. eugenii, however, the anterior premolar (pm^) was much more advanced in its development and consequently the diastema was much smaller. I account for the absence of these teeth by the early enlargement of pmt before the maxilla is elongated, and in consequence this tooth overshadows the region of the 1st and 2nd premolars and abstracts the matter and power of growth which would otherwise fall to their share. Kiikenthal, in his description of the premolars of Bidelphys, describes what he considers to be a rudiment of the successional tooth as attached to the enamel-organ of the 1st premolar, but he found no trace of the missing premolar, nor any rudiment of a successional tooth to the 3rd premolar, while he describes the functional successional tooth as being developed from the enamel-organ of the 4th premolar. N o w m y investigations among the Kangaroos show that in them the one functional successional tooth is never by any chance developed from the 4th premolar, and although it displaces that tooth along with the third, it is not the representative of the same (the 4th pm.) in the 2nd dentition. If this is the case, then the conclusions of Owen, Grervai3, Flower, and Thomas break down as far as the Macropodidae are concerned. Having shown that this tooth is not the successor to the 4th premolar, it remains to decide if possible what its real significance is. Judging from its relation as seen in JEpyprymnus alone, I should have concluded that it really represented the successor of the 3rd premolar ; but the embryos of this form and also those of the various species'of Macropus which I have examined were all too old to show the actual origin of this replacing tooth. The only form in which I could observe the first origin of this so-called successional tooth was in Petrogcde, and here, as 1 have described above, this tooth arises independently of the 3rd and 4th premolars from the dental ridge connecting these two teeth. Its position there certainly suggested that it represented a tooth intermediate between the 3rd and 4th premolars, and belonged to the same series as themselves, owing its subsequent position internal to and deeper in the g u m than these teeth to the more rapid growth and earlier development of the latter, wdiereby this intermediate tooth is displaced and retarded, so that it assumes all the relations of a tooth of the second dentition. The first stage in this change is well seen in the youngest embryo of M. giganteus. This tooth often takes on a secondary connection with the adjacent premolars ; thus in Petrogcde it becomes connected with the 4th premolar, while in Macropus and Mpyprymnws it is related to the 3rd. It is interesting to note that in Perameles the large supposed successional tooth is quite distinct in origin from the small 4th premolar which is shed ; it is in fact formed from the dental |