392

MARSUPIALS.

principally distinguished from one another by the different size andshape of the permanent premolar tooth, or that which displaces andsucceeds the deciduous molars in the vertical direction. This toothis displayed in its closed alveolus in both specimens(l) in whichsituation, notwithstanding their superiority of size over the largestexisting Kangaroos, I was led to seek for it, by observing the sharp,unworn summits of the crowns of the molar teeth and other signs ofimmaturity in the fossil specimens.

The total number of molar teeth successively developed in thegreat extinct Kangaroos is the same as in the existing species, viz :two deciduous molars, one premolar, and four true molars : and thepermanent series of five appears to have been longer retained than inthe large existing species.

The true molars of the upper jaw in the Macropus Titan differfrom those of the Macr. Atlas and of all the existing species in havinga well-developed ridge at the back part of the base of the crown inaddition to the two principal transverse eminences and the anteriorbasal ridge. In the Macropus Atlas the posterior basal talon of theupper molars is much smaller, the crown broader, especially itsanterior division, and the ridge connecting the two transverse emi-nences is shorter and more simple. In the lower jaw r the molarslikewise present modifications characteristic of the two species : thoseof the Macropus Titan have no posterior basal ridge, but the anteriorone is longer, as is also the ridge connecting the two chief transverseeminences ; and the antero-posterior extent of the crown is greater inproportion 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 infig. 1.

In the lower molars of Macropus Atlas the posterior talon existsin the same rudimental state as in the upper jaw, and the anteriortalon is shorter than in the Macr. Titan, as is shown in the rightpenultimate 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 originallydescribed by me in Sir T. Mitchell’s “ Expeditions into Australia.” 8vo. 1838, Vol. ii, p. 361,PI. 29.