452 ME. M. F. WOODWARD OX MAMMALIAN DENTITIOX, [May 2, 



PetROGALE PEXIC'ILLATA. 



Examined 4 embryos, either by clarification or sections, or both. 

 Specimens examined measured from snout to tip of tail respectively 

 8'5, 9*5, 11"5, 14-5 centims. 



D. f. of adult according to Thomas (12), I. ^'"' C. q, P. q^W^' 



■^*^' 1 . 2 . 3 . 4- 



Fig. 1, Plate XXXY., represents the entire jaw clarified with the 

 teeth in situ. 



At this stage it will be seen that there are present in the upper 

 jaw in all 10 teeth, 6 being in the premaxilla and 4 in the 

 maxilla, while in the lower jaw there are only 6 teeth. In the 

 upper jaw, only three out of the 10 were calcified, viz., i^, i^, i\ 

 fig. 1, all being situated in the premaxilla ; from their minute size 

 and advanced calcification these are obviously vestigial structures, 

 the three functional incisors of the adult being at present 

 represented only by their enamel-organs, of which the first two are 

 very large (t' and i*). 



In the maxilla the germs of 4 teeth are present, the 1st, from its 

 proximity to the premaxillo-maxillary suture and the large 

 diastema which separates it from the remaining teeth, evidently 

 represents the canine, while the others represent the 3rd and 4th 

 premolars and the 1st molar. 



In the mandible we find the large functional incisor (i.^) of the 

 adult slightly calcified, and on either side of this a small rudi- 

 mentary but well-calcified functionless tooth (i^ and i^) ; behind 

 these we find the 3rd and 4th premolars and the germ of the 1 st 

 molar. 



The Incisors. 



The most striking feature in this Petrogale is the presence of 

 three additional vestigial incisors in the upper jaw and two in the 

 mandible, — a fact not hitherto recorded. The presence of these 

 teeth naturally enables us to homologize the functional incisors of 

 the adult with those of the Polyprotodont Marsupials. 



Here I would direct attention to a source of error which might 

 creep in through the examination of clarified preparations of the 

 jaw only, for it will be seen that the 1st rudimentary upper 

 incisor is apparently situated in front of the 1st functional one; 

 and it was only by means of a very carefully prepared series of 

 sections taken at right angles to the premaxilla, starting parallel 

 with the premaxillary suture and changing the plane of the 

 sections as one passed outwardly, so that the plane of the last 

 section was at right angles to that of the first, that one was 

 enabled finally to determine which of these teeth was really the 

 1st incisor. 



Fig. 2 is a wax model constructed from a series of sections by 

 His's method; this shows that the enamel-organ of the 1st large 

 and at present undifferentiated tooth is connected with the gum 

 nearer to the premaxillary suture than that of the small rudi- 



