1893.] MR. M. F. AVOODAVARD O^ :MAMMALIAX DEyTITIOX. 459 



of the latter, they somewhat overlap the lamina connecting the two 

 and it in consequence becomes displaced to the inner side of these 

 structures. So much does the 4th premolar grow forward with, age 

 that it appears as if this lamina was a doAAngrowth from the inner 

 side of the enamel-organ of ^j»!* ; this, however, is really not the 

 case, the lamina is morphologically in front of that tooth and only 

 attains a secondary connection with it. 



A careful examination of the swollen part of this lamina (fig. 16, h) 

 at this more advanced age show s that we are undoubtedly dealing 

 with a developing tooth. Xoaa- the only tooth a\ hich develops in 

 this situation is the one successional or replacing premolar of the 

 Marsupials, and this is usually regarded as being the derivative of 

 the 4th premolar, which latter, as far as I have observed, in Petrogcde 

 never develops a successional tooth at auy time. The successional 

 tooth is here deAcloped from a dental lamina situated between pm^ 

 and j)?»*, but may afterwards attain a secondary connection with 

 the latter tooth, but does not represent its true successor. The facts 

 appear to me to strongly suggest that this tooth represents a 

 premolar belonging to the same series as i^m^ and jym^ Avhich has 

 been retarded in its de\elopment and in consequence does not cut 

 the gum until long after the others. The further consideration of 

 this point I shall leaA^e till I have described the condition in the 

 other Maeropids here dealt AA-ith. 



The condition in the lower jaAv is the same as that in the upper. 



The Molars. 



^\niile investigating the condition of the developing molars in 

 the Kangaroos I naturally sought to confirm Kiikenthal's sugges- 

 tion that the molars belonged to the first dentition ; this, however, 

 I have been unal)le to do. In fact, if any reliance is to be placed 

 on these downgrowths from the enamel-organs of the deA^eloping 

 teeth or from the dental lamina, then I must assert that the 1st 

 and 2nd molars of the Macropodida) belong to the 2nd dentition, 

 and that therefore Kiikenthal's original suggestion does not hold 

 good for the Marsupials in general. 



The great difficulty AA-hich we find in connection with the interpre- 

 tation of the molars arises from the shortness of the jaws, so that 

 according to the age Ave find the most posterior molar as yet 

 formed developing side by side with the penultimate one. This led 

 Kiikenthal into a mistake, Avhich he has since pointed out to me. 

 In his paper (no. 5) he figures the 2nd molar of DidelpJiys 

 Avith a rudimentary successional tooth ; this is really, as he now 

 believes, the rudimentary 3rd molar developing side by side with 

 the 2nd. He, however, describes Avhat he believes to be a trace 

 of a successional tooth in coiniection with the 1st molar, but 

 unfortunately he does not figure this structure. I have found no 

 trace of any such structure either in D'uldjiJti/s or in I\Iacropodidae, 

 and therefore see no reason to believe that the molars an^ in any 

 sense referable to the 1st dentition. 



On the other hand, sections taken across tlie jaAv of Pelroffale, 



