392 MARSUPIALS. 



principally distinguished from one another by the different size and 

 shape of the permanent premolar tooth, or that which displaces and 

 succeeds the deciduous molars in the vertical direction. This tooth 

 is displayed in its closed alveolus in both specimens(l) in which 

 situation, notwithstanding their superiority of size over the largest 

 existing Kangaroos, I w^as led to seek for it, by observing the sharp, 

 unworn summits of the crowns of the molar teeth and other signs of 

 immaturity in the fossil specimens. 



The total number of molar teeth successively developed in the 

 great extinct Kangaroos is the same as in the existing species, viz : 

 two deciduous molars, one premolar, and four true molars : and the 

 permanent series of five appears to have been longer retained than in 

 the large existing species. 



The true molars of the upper jaw in the Macropus Titan differ 

 from those of the Macr. Atlas and of all the existing species in having 

 a well-developed ridge at the back part of the base of the crown in 

 addition to the two principal transverse eminences and the anterior 

 basal ridge. In the Macropus Atlas the posterior basal talon of the 

 upper molars is much smaller, the crown broader, especially its 

 anterior division, and the ridge connecting the two transverse emi- 

 nences is shorter and more simple. In the lower jaw the molars 

 likewise present modifications characteristic of the two species : those 

 of the Macropus Titan have no posterior basal ridge, but the anterior 

 one is longer, as is also the ridge connecting the two chief transverse 

 eminences ; and the antero-posterior extent of the crown is greater in 

 proportion to its breadth, than in the Macropus Atlas : these charac- 

 ters are shown in the figure of the left penultimate molar, PI. 101, 

 fig. 2 ; the greater thickness of the transverse ridges is shown in 

 fig. 1. 



In the lower molars of Macropus Atlas the posterior talon exists 

 in the same rudimental state as in the upper jaw, and the anterior 

 talon is shorter than in the Macr. Titan, as is shown in the right 

 penultimate molar* in PI. 101, fig. 4. The upper premolar of the 



(1) They are now preserved in the Museum of the Geological Society, and were originally 

 described by me in Sir T. Mitchell's " Expeditions into Australia." 8vo. 1838, Vol. ii, p. 361, 

 PI. 29. 



