1893.] ME. 11. r. WOODWAED ON MAilJLUilAlf DENTITIO?)^. 455 



If I am right in concluding that these three teeth belong to the 

 same dentition, then the 2nd functional incisor of the adult is in 

 reality the 4th incisor, this small calcified tooth is the 5th, and the 

 3rd adult incisor must consequently represent a 6th incisor. 

 Unfortunately, although possessing several specimens of fhis 

 interesting form they were all too young to show the further develop- 

 ment of these thi'ee last incisors, and in order to completely settle 

 the question as to the relation betrvveen these teeth I was obliged 

 to refer to other Macropids of which I possessed older examples. 



In both Macropus girjanteus and M. hrachyurus the second adult 

 incisor sho\AS a fairly well-marked rudiment of the replacing tooth 

 in the form of a cellular downgrowth from the inner side of the 

 neck of its enamel-organ, thus showing that this tooth itself 

 belongs to the 1st dentition. A similar condition is observable in the 

 3rd functional incisor of M. gic/anteus and of M. euc/enii (figs. 5 and 

 6, i*'), which must also be referred to the 1st dentition, and not, as 

 Rose believes, to the 2nd. In both these forms the small calcified 

 incisor (i') is present and situated between the two larger ones ; 

 in 21. girjanteus^ it is extremely vestigial and only to be recognized 

 \vith difficulty ; but in M. eugenii it is, on the contrary, very large 

 and well developed. In no case, however, did it show any indi- 

 cation of a replacing tooth as seen in the 1st rudimentary incisor 

 of Petrogale ; so that it is impossible to say for certain to which 

 dentition it was to be referred, but, judging from its analogy with 

 the other vestigial teeth, it should belong to the 1st dentition. 



We find therefore in the upper jaw of Petrogale imiicillata 6 

 incisors, three of which are very small and obviously disappearing, 

 and three which we recognize as the incisors of the adult : these 

 teeth are all referable to the 1st (or milk) dentition, the 1st, 2nd, 

 4th, and 6th all possessing at some period in their development, 

 either in Petrogale or in some other allied Macropid, rudiments of 

 the 2nd or replacing teeth. 



The adult incisors are the 1st, 4th, and 6th, the 2nd, 3rd, and 

 5th being in a vestigial condition and completely reabsorbed 

 almost before the former become calcified. 



Rose's suggestion concerning the relation of the 3rd adult incisor 

 {i^) is not borne out by the study of its development, fig. 5 

 distinctly showing that in M. giganteus this tooth belongs to 

 the 1st dentition, and not to the 2nd as he stated for that species. 

 His conclusions were based solely on the fact that the 3rd incisor 

 cuts the gum so much later in life than the anterior teeth, Thfe 

 explanation of this arrangement becomes self-evident when one 

 compares the young skull (fig. 29) with that of the adult : it \vill 

 be seen at once that the premaxilla is very small in the former in 

 proportion to the teeth, and is consequently far too short to 

 accommodate the three large incisors of the adult at the same time, 

 the teeth being extra large for a f cetus owing to the fact that they 

 are the adult teeth and are not replaced by a second set as in most 



^ I have since cut a younger M. giganteus, and find this tooth is very large; 

 so that in the specimen above mentioned it must have been largely reabaorbea. 



31* 



