PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
537 
In the older specimen (ib. fig. 11) the two lobes of m 2 are half worn down; the 
abraded surface of each is gently bent with the concavity forward ; the transverse extent 
of such surface is 1 inch 7 lines ; the fore-and-aft extent of the tooth is 2 inches 4 lines. 
The abraded surfaces slope from before downward and backward. 
In the last molar of the same lower jaw the summit of the hind lobe, on which a 
narrow tract of dentine is exposed, measures 1 inch 4 lines in transverse extent, that 
of the more worn front lobe being 1 inch 6 lines. The transverse extent of the base 
of each lobe is the same, viz. 1 inch 7 lines. The summit of the hind transverse ridge 
(g) is continuous with a short low rising upon the back part of the hind lobe. The 
antero-posterior extent of the tooth is 2 inches 5 lines. 
There is less difference between m 2 and m 3 of the lower jaw than in the upper one. 
Owing to the direction of the plane of attrition, the front surface of each worn lobe is 
higher than the back surface ; the front lobe, when unworn, rises a little higher than the 
back one. The fore part of each tooth rises more abruptly, and in a greater degree 
above the back part of the tooth in advance ; thus the line of attrition of the entire 
molar series is zigzag. The general curve of the grinding-surface of the four molars is 
slightly concave from before backward, as, above, it is convex. The contour of the outer 
sides of the lower series of molars is slightly convex ; that of their inner sides is almost 
straight. 
In the mandible belonging to the skull (Plate XXXV. fig. 1) the outer part of m 2 
is worn to its base, and a larger proportion of m 3 alone remains in the left ramus*. 
The fore-and-aft extent of m 3 is 2 inches 5 lines : the same extent of the abraded 
surface of the front lobe is 9 lines, its transverse extent being 1 inch 7 lines. The enamel 
at this part of the tooth is fully a line in thickness. 
In a fragment of the left mandibular ramus of an old Diprotodon are the last two 
grinders (Plate XL. figs. 17, 18), similarly worn down but better preserved. In m 2 
(fig. 17) a portion of the enamel at the inner end of the valley ( e ), and the enamel of the 
hind part of the base of the hind lobe with the contiguous basal ridge alone remain ; the 
rest of the surface is polished dentine and osteo-dentine with the external cement. 
In rn 3 (fig. 18) the enamel is worn away from the fore and outer part of the front lobe; 
the front basal ridge is rounded off; the outer boundary of the valley connecting there 
the front and hind lobes is smoothed down, and the middle of the hind transverse 
ridge is touched. Both lobes are worn down nearly to the bottom of the valley. 
At the middle of each of the smooth concave plates of dentine, a central tract of osteo- 
dentine (o) is defined. The antero-posterior extent of m 2 is 2 inches 4 lines ; that 
of m 3 is 2 inches 5 lines ; the greatest transverse diameter in each is 1 inch 7 lines. 
This Diprotodon had probably died of old age. The outer alveolar border has grown 
upward with the rise of the fangs and base of the teeth to bring them into grinding con- 
tact with those of the upper jaw. 
In the crown of the last molar not wholly emerged from the formative alveolus of the 
* The form and position of these teeth are given, in outline, from better preserved and less worn specimens. 
4 d 2 
