194 
MACROPODID/E. 
Hypsiprymnm campestris: the prcmolar tootli is larger, and 
almost always lias numerous distinct vertical grooves both on 
the outer and inner sides*. The true molar teeth differ, in 
being successively smaller from the foremost to the last, and 
the crowns of these teeth have the two principal transverse 
ridges much less developed. A more accurate idea of the struc¬ 
ture of the unworn grinding surface of the molar tooth of a 
Hypsiprymnttfi (see Plate 10, figs. 5 and ba) would be con¬ 
veyed by describing them as of a quadrate form, and 
presenting four equidistaut blunt tubercles, which are joined 
in pairs by transverse ridges, hut with these ridges less 
elevated than the points of the tubercles : there is a slight 
trace of the hand of the tooth both on the front and back of 
each molar, as in Macropus. The hindermost molar is 
generally small, and sometimes almost round 1 . 
On Plates 6, 8, and 1 0, are represented the skulls of the various 
species of Hypsiprymnus, or Rat-Kangaroos. Since the last 
tooth to make its appearance in these animals is the permanent 
premolar, it very frequently happens that the skulls in collections 
do not show this tootli. At different ages previous to the animals 
having attained maturity, the skull will present two molars 
belonging to the milk, or first series, and from one to a perfect 
scries of the true molars. Wlien all the true molars are just 
developed, and there are six molars on each side of either 
jaw, it will be found that the foremost of each row corresponds 
almost perfectly in structure with the permanent premolar, ex¬ 
cepting that it is of rather smaller size; and the second tooth, 
which resembles a true molar, excepting in being rather smaller, 
are teeth of the first series; for they will be replaced in the 
vertical direction by another tootli, and this is the permanent 
premolar. The skulls, tigs. 2 and 4, of Plate 10, belonging to 
animals which were not quite adult, present the condition of 
1 Cases occur in which the hist molar tooth is absent; and, what is more 
extraordinary, I have observed an extra molar tooth on each side of the 
upper jaw, in a species of Hypsiprymnus. 
