532 PEOFESSOE OWEN ON THE FOSSIL MAMMALS OP AUSTEALIA. 



A cursory comparison of the two foregoing specimens suggests that m 1 (fig. 3, Plate 

 XXXVII.) may have been destined to be pushed out by a vertical successor, which, in 

 place in the larger specimen [m 1, fig. 2, Plate XXXVIII.), shows of course a less degree of 

 abrasion. But this is not the case. I have in vain sought for evidence of any premolar, 

 in either upper or lower dental series of Diprotodon : it differs from Macropus and resem- 

 bles Phascolomys in this particular. All the teeth, like the last three grinders in the 

 type diphyodont dentition, belong to the first set. The variety as to degree of attrition 

 in molars of the same series is due to some modified habit of mastication : the difference 

 in respect of size I ascribe to sex, the smaller grinders belonging to the female, con- 

 comitantly with a general inferiority of bulk, as is seen in Macropus. The following 

 admeasurements exemplify the diff'erence of size in molar teeth, which is probably 

 sexual : — 



Diprotodon. 



< " N 



Male. Female, 

 in. lines. in. lines. 



m 3. Antero-posterior diameter 1 10 1 7 



Transverse posterior diameter (base of front lobe) .1 11 1 6 



m 4. Antero-posterior diameter 2 3 2 0 



Transverse posterior diameter 2 0 1 7 



m 5. Antero-posterior diameter 2 4 2 1 



Transverse posterior diameter 2 0 1 7^ 



Antero-posterior extent oim 3, «i 4, «i 5 . . . . 6 0 5 10 



The forms and proportions in which the four constituents of the molar teeth of Dipro- 

 todon are combined, are exemplified, in the vertical longitudinal section of the last three 

 upper grinders, in Plate XLII. fig. 1. The enamel {e) gains thickness as it recedes 

 to a certain extent from the summits of the lobes, giving more resistance or grinding- 

 power as the tooth wears down ; but the enamel thins again at the base of the lobe ; it 

 gains a little more thickness as it is reflected, so to speak, over the basal ridges, beyond 

 which it extends from three to four lines before thinning off", and ceases upon the body 

 of the tooth before its division into fangs. The usual general direction of the dentinal 

 tubules is well displayed, as in most fossil teeth. As the dentine becomes exposed and 

 abraded, the pulp-cavity is defended by the coarser calcification of the remaining matrix 

 near the field of abrasion, and from 2 to 3 lines of osteo-dentine is interposed between 

 that field and the pulp-cavity. In each lobe of the tooth most worn {in 1) the cavity is 

 reduced to a linear trace. In the anterior lobe of m 2 it is more expanded ; and it 

 retains width in both lobes of m 3. In each tooth the pulp-cavity has received a 

 lining of dark-coloured spar in the course of fossilization. The cement is thickest upon 

 the back part of the hind root (c), whence it extends upon the posterior basal ridge : 

 this partial excess of cemcntal development assumes a characteristically definite figure 

 in such sections as the one described. 



The lower incisors (Plates XXXV., XLI. & XLII. i; Plate XXXIX. figs. 4, 5, G) are 



