458 :mr. m. f. woodward oif mammalian det^titiow. [May 2, 



so that one can only suggest by a comparison with the 1st of these 

 teeth that it is to be referred to the 1st dentition. 



The Canines. 



Owen (p. 291) and Plower have already pointed out the 

 existence, in the mammary foetus and the adults of some 

 Kangaroos, of vestigial upper canines, so that their presence 

 here in all the Macropodidae (save ilf. hrachyurus) which. I have 

 examined calls for no special comment. 



The dental sac of this tooth is of considerable size and situated 

 close to the premaxillo-m axillary suture, a considerable space 

 intervening between it and the premolars behind. 



In the dried skull (7 cm. long) of an advanced mammary foetus 

 of Petrogale xantliopus the cauine was present on either side as an 

 exceedingly minute pointed tooth in a distinct alveolus (fig. 29, c) '. 

 It seems probable that this tooth is shed, not reabsorbed, being 

 pushed out of the gum by the development of the enormous 3rd 

 incisor which is situated immediately internal to it. 



There is no trace of a lower canine in any Macropid which I 

 have examined. 



The Premolars, 



The dental lamina is more or less continuous between the 

 canine and the most anterior upper premolar {pm^), but at the 

 same time it is very irregular, showing several curious swellings 

 (fig. 12) ; these, however, are so irregular that T hesitate in ascribing 

 any importance to them, but it is just possible they may represent 

 the missing 1st and 2nd premolars. 



The 3rd premolar (fig. l,pm^) shows as a fair-sized dental sac 

 with a well-developed enamel-organ and pulp, but at present there 

 is no trace of calcification, its enamel-organ remains attached to the 

 small dental lamina whose free end projects slightly into the 

 surrounding tissue (fig. 15) ; this structure, which might possibly 

 be interpreted as a rudimentary successional tooth, disappears as 

 we ti'ace this tooth backwards in the sections, and soon also does 

 the swollen portion of the enamel-organ of the 3rd premolar itself, 

 but in its place, and directly continuous with it, we find a strongly 

 developed club-shaped mass of cells which run in form of a lamina 

 between the 8rd and 4th premolars, connecting tlieir enamel-organs 

 and bearing a definite swelling in the middle (fig. 19, ppm). A 

 section through this region shows (fig. 16) that we are dealing 

 \vith an incipient enamel-organ, the centre of which is already 

 differentiated into the typical stellate tissue. 



The 4th premolar, which is vei-y lai'ge though not calcified, is 

 still connected by the dental lamina with the gum, but shows no 

 trace of a successional tooth unless the small process of the enamel- 

 oi'gan (fig. 18, p) be interpreted as such. 



In a slightly older embryo the same condition was observed, but 

 owing to the development of the 3rd and 4th premolars, especially 



' In fig. 29 the canine is represented as lying in the suture between the pre- 

 raaxilla and the maxilla, in reality it ie surrounded by a thin layer of the latter, 



/ 



