1893.] MB. M. F. AVOODWAHD OJf MAMMALIAK DEIfTITION. 467 



canine and the third premolar, so that one might reasonably expect 

 to find some trace of these teeth ; in M. hrachi/urus and 31. eugenii, 

 however, the anterior premolar (jym^ 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 enlarge- 

 ment of pm^ 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 

 other\\ise fall to their share. 



Kiikenthal, in his description of the premolars of Bidelplvjs, 

 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 -Ith premolar. Xow my 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, theu the conclusions of Owen, G-ervais, 

 Flower, and Thomas break down as far as the Macropodidse 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 ^pijjjrijmnus 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 Macivjms 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 Petrogale, 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 gum than these teeth to the more 

 rapid growth and earlier development of the latter, whereby 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 Petrogale it becomes connected with 

 the 4th premolar, while in Macropus and JEjiyprymnus 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 



