470 MR. M. F. WOODWARD OX ilAMM.iilAX DEXTITIOX, [May 2, 



not this retardation of the 4th premolar as seen in the Macro- 

 podidaj be the tirst step in the formation of the permanent set', 

 which mav afterwards take on a secondary connection with the 

 teeth of the 1st dentition? In Amphilestes there are 12 or 13 

 cheek-teeth present, and no evidence of the presence of two 

 sets of teeth. ]\[ay not the five posterior ones represent the five 

 molars (Bettonr/ia), while the first 8 might be supposed to give rise 

 to the 8 premolars (4 milk and 4 permanent), and by the retar- 

 dation of each alternate one the condition in the Placentalia might 

 be brought about, the 2nd, 4th, 6th, and Sth being retarded and 

 displaced to form a second or replacing set, whilst the 1st, 3rd, 

 5th, and 7th develop early and are replaced by tlie former? These 

 teeth, which were originally distinct, may have acquired a secon- 

 dary connection with the tooth in front, as seems to have been the 

 case with the '3rd premolar of the Kangaroos and the one replac- 

 ing tooth, this latter in those forms displacing both the 3rd and 

 so-called 4th premolar. In others it, however, only displaces the 

 so-called 4th premolar, owing to the latter having, through its 

 enlargement, acquired a connection with the replacing tooth, as 

 in Didelphys ; or, owing to the reduction in size of the 4th 

 premolar, as in Perameles and Thi/lacinus, the supposed replacing 

 tooth is able to cut the gum in its more normal position and 

 displaces the reduced tooth behind. 



If these various and often minute cord-like downgrowths of 

 the dental lamina are to be interpreted as representing rudi- 

 ments of teeth, as seems probable from comparison with the 

 known rudiments of the 1st or 2nd dentition in other mammals, 

 then we find that in the Kangaroos the incisor teeth all belong to 

 the 1st dentition, that the relations of the canine are uncertain, 

 that the premolars probably belong to the 1st dentition, whereas 

 the molars, or at any rate the 1st, belong to the 2nd dentition. 



This last statement is a reversion to older ideas as to the 

 relation of these teeth, held by all odontologists prior to the 

 appearance of Kiikenthal's paper, wherein he formulated the theory 

 that the molars belonged to the 1st dentition. As I have already 

 pointed out, he has retracted part of his statements on this point, 

 and I have been unable to confirm his views as to the 1st molar 

 in Diddphys, while in the Macropodidse I have apparently found 

 exactly the reverse condition^. 



I should suggest by way of explanation as to the presence of 

 the permanent molars in- a dentition which was otherwise entirely 

 composed of milk-teeth, that owing to the shortness of the jaws 

 the molars were formed very late, and owing to the inability to 

 find room for two sets of what are naturally large teeth, the 1st or 

 milk dentition, as the least important, became suppressed, and is 

 only seen as a slight rudiment attached to the least modified molar, 



^ Similar to the condition seen in the Monitor amongst Reptiles. 



^ For the present I leave out of consideration Leche's (6) account of the con- 

 dition and homology of the molars of Erinaceus, as I have not yet finished my 

 observations on the molars of the Placentalia. 



