1893.] 5tR. M. F. WOODWAED ON MAMMALIAN DENTITION. 457 



incisors 4 and 5, in the position of the rudimentary incisor 5 of 

 Petrogale Bndi of the large diastema in the adult Perameles, and this 

 possibly represents a 6th incisor. From this I conclude that the 

 Marsupials originally possessed 6 pairs of incisors in the upper jaw, 

 the first of which to disappear being no. 5 in the Polyprotodonts, 

 while in the Macropodidte nos. 2, 3, and 5 are wanting ; here, 

 however, no. 5 is less reduced than nos. 2 and 3, this being due to 

 the fact that the large functional incisors nos. 1 and 4 develop 

 early and consequently cause a corresponding early absorption of 

 nos. 2 and 3, while owing to the late development of no. 6 the 

 smaU no. 5 persists for a considerable period. 



TJie Lower Incisors. 



In the adult Petrogale and in all the Macropodidse there is but 

 one incisor in either half of the lower jaw, which tooth according 

 to Thomas is to be identified either with no. 1 or 2 of the Poly- 

 protodonts. 



In the young Petrogale there are, ho^^ever, no less than three 

 teeth situated close on either side of the mandibular symphysis. 

 Fig. 1 shows one set of these teeth in situ : it will be seen 

 that two of them (i, and /J are very small although fully 

 calcified, consisting even at this early period of both enamel and 

 dentine. The remaining one is Aery large and may be shown to 

 be the developing single incisor of the adult. The position of the 

 two minute teeth close the tip of the large incisor {i^ shows the 

 former to be undoubted incisors. 



The examination of a series of sections through the mandible 

 showed that the most anterior of these three teeth is the minute 

 tooth ij, fig. 9, and we may call this tooth provisionally the first 

 lower incisor. This tooth is so well calcified that, in spite of its 

 small size, we must conclude that it is fully formed, and from an 

 examination of a series of stages I have come to the conclusion 

 that it does not cut the gum but is probably reabsorbed. One of 

 the most interesting points in connection M'ith this tooth is the fact 

 that fi'om the posterior or internal side of its enamel-organ there 

 arises a long cord-like downgrowth with an enlargement on its 

 deep-seated extremity (figs. 9 and 10, i^), which at a later stage 

 becomes slightly indented below, and has all the relations of an 

 undeveloped permanent tooth. This small vestigial incisor (i\) is 

 thus proved to belong to tlie 1st dentition ; and this fact further 

 clears up any doubt which might exist as to its relation to the 

 functional incisor (ij, to which it is very closely approximated, 

 but to which it is in no way related. 



Judging from the analogy of the other incisors, I should conclude 

 that the functional incisor also belongs to the 1st dentition, but I 

 have not been able to trace any very definite downgrowth from 

 the inner side of its enamel-organ in this species. 



The 2nd small tooth (ij (? morphological 3rd incisor) is more 

 vestigial than the first, and shows no trace of a successionalt ooth. 



