4fi4 \n?, 7*r. r. woomvAwn ox MAMMALTAX dextittox. [May 2, 



^PTPBTMNUS RFFESCEXS. 



Two specimens, measuring 125 mm. and 195 mm. long respec- 

 tively. 



In this Kangaroo-Eat no trace of the reduced incisors was 

 observed, the three functional ones being well developed, the first 

 and the last showing traces of an internal downgrowth from the 

 dental lanima similar to that seen in the true Kangaroos. 



The jaw is hei'e very short, so that the last incisor, the canine, 

 and the anterior end of the first premolar appear in the same 

 section. The last-mentioned tooth is very large and extends 

 through a great number of sections ; towards its posterior end, 

 and just as the 4th premolar is appearing on its external side 

 (fig. 24), a mass of cells is noticed lying above it ; this mass of cells 

 is connected with a long epitheloid cord which is applied to the 

 inner side of the dental sac of the 3rd premolar \ the two having 

 very much the same relation as that seen between the dental sac 

 of a milk-tooth and its so-called permanent successor. 



The condition seen in the lo^er jaw is very similar to that in the 

 upper, except that the swollen portion of the germ of the so-called 

 successional tooth is situated rather farther back, as in most other 

 Macropids, owing to the backward extension of the large incisor. 



If one was able to study the development of the functional 

 successional tooth in this form only, one could not help concluding 

 that it was developed side by side with the 3rd premolar, and that 

 it really represented the successor of that tooth and not that of the 

 4th premolar as has generally been believed hitherto. 



Bettongia lesueuri. 



One embryo, about 180 mm. long. 



The teeth of this foetus were in an advanced stage of calci- 

 fication and no traces of the supposed rudimentary successional 

 teeth were to be observed. 



There were four pairs of incisors present in the upper jaw, 

 viz. ?\ ?'*, i% and i'^ ; of these the first two and the last Mere very 

 large, while i^ Mas minute, sooiewhat irregular, and undergoing 

 absorption. All traces of i' and i^ had disappeared. Xo trace of 

 additional incisors was observed in the lower jaw. 



The upper canine in these Kangaroo-Eats persists in the adult 

 as a large tooth, and in conformity M-ith this we find that in the 

 embryo this tooth is larger than in the true Kangaroos and M'ell 

 calcified. The 3rd premolar is very large, and already shows its 

 characteristic shape, it overlaps the canine in front and nearly 

 reaches the 4th premolar behind. The one successional tooth is of 

 considerable size and slightly calcified ; its enamel-organ is con- 

 nected by a long strand of epithelium with the extreme posterior 



^ In the figure the cord of epithelium connected with the developing premolar 

 (ppm) is represented as coming into too intimate connection witli the enamel- 

 organ of the 3rd premolar ; it should in reality be only applied to the side of 

 that structure, and not fused with it as appears from the drawing. 



