PROFESSOE OWEN ON THE FOSSIL MAMMALS OF AUSTEALIA. 



537 



In the older specimen (ib. fig. 11) the two lobes of m2 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 

 (ff) 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 in 2 and w 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 to 2 

 is worn to its base, and a larger proportion of w 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 Biprotodon 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 m 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 Biprotodon 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 



