MARSUPIALS. 397 



the teeth than in the Thylacine, the Wombat or the Kangaroo : 

 the trunks resolve themselves into penicilli of minute radiating 

 branches, which interlace and form a rich net- work at the boundary- 

 line between the dentine and enamel. 



In the large lower incisors of the Kangaroo (PL 102, Fig. 1 , 

 d d.) the dentine is very compact, and the tubuli extremely 

 minute : their diameter is ^^^^ of an inch, and their intervals equal 

 three of these diameters. The tubes radiate with elegant secondary 

 undulations, from the central pulp-cavity to the periphery. The dicho- 

 tomous divisions of the tubuli are sparing until they approach their 

 terminations : the extremity of one of the tubes magnified 600 

 diameters, is represented at PI. 102, fig. 2 : the lateral branches 

 are unusually minute, numerous, and short, giving the tubuli 

 a pilose appearance when brought into view by an adequate 

 magnifying power. The terminal branches of the tubuli open 

 into minute irregular cells, forming a thin boundary layer between 

 the dentine and enamel. The fibres of the enamel which in- 

 vest the crown of the large lower incisor, are likewise un- 

 usually minute ; viewed in transverse section, as in PI. 102, e e, they 

 describe an abrupt curve at their commencement, and then proceed 

 in a nearly straight course to the outer surface ; but at the trenchant 

 margins of the tooth their general course is curved, and they decus- 

 sate one another as represented in the figure ; some of the enamel 

 lines at this part seem as fine as the dentinal tubes. An extremely 

 thin layer of cement may be traced over the basal half of the 

 enamelled crown ; it is soon worn away from the remainder ; the 

 cement increases in thickness where it invests the fang. 



The dentine of the scalpriform incisor of the Wombat is charac- 

 terized by a few short medullary canals continued from the long 

 conical vascular pulp. The dentinal tubuli are rather larger than 

 in the procumbent incisor of the Kangaroo, but are separated by 

 still narrower interspaces ; they dichotomize very sparingly, but 

 send off many fine branches from the side of the primary curve 

 of the tube which is directed towards the crown of the incisor. 

 The boundary layer of cells, in which the terminal branches of 

 the tubuli open, is thicker than in the Kangaroo. The layer of 



