530 



PEOFESSOE OWEN ON TKE FOSSIL MAMMALS OF AUSTEALIA. 



working surfaces of the four molars describes a slight convexity downwards (Plate 

 XXXVIII. fig. 1) ; the exterior line is also slightly convex (ib. fig. 2) ; the interior line 

 is concave in a less degree ; the right and left series are moderately convergent anteriorly. 

 The interspace between the hind lobes of the last ^molars (m 3) is 4 inches 1 line ; that 

 between the front lobes of the first molars (d 4) is 3 inches 1 line ; these dimensions give 

 the breadth of the palate between the right and left teeth above symbolized. 



All the molars in place have an enamelled crown divided into two transverse lobes 

 (Plates XXXVII., XXXVIII. a, I), with accessory ridges (/, g), and are inserted by 

 cement-clad contracting roots as in Macropus. The summits of the transverse lobes are 

 abraded in all the molars of the specimens figured ; but least so in the hind lobe of the 

 subject of m 3, fig. 2, Plate XXXVIII. 



The socket of the first small molar (fig. 1, 3) is partially preserved in the entire skull ; 

 it consists of two cavities, the hindmost the largest, the tooth having only two roots. 

 In the subject of Plate XXXVII. figs. 1 & 2, the crown of <Z 4 is ground down nearly to 

 the bottom of the cleft (A) ; the fore-and-aft extent of the grinding-surface is 1 inch 2 

 lines ; its transverse extent across the hind lobe is the same. Across the fore part of the 

 base of the tooth is a low ridge (/}, to the level of which the anterior lobe (a) is almost 

 worn. The corresponding ridge at the back part of the tooth is continued along both 

 the outer and inner borders {g^ g) of so much of the back part of the hind lobe as is not 

 ground down; the whole of the surface projects beyond the level of the worn surface of 

 the following molar (fig. 1, m). The transverse cleft is deepest at its outer and inner 

 ends ; a ridge of enamel descends from each of these ends of the anterior lobe, and, 

 meeting a corresponding projection of the oppposite lobe, it partially closes the entry 

 of the valley. 



The anterior basal ridge is strongly developed in m 1, especially at its inner end ; the 

 interspace between it and the anterior lobe widens toward the inner side of the tooth (/). 

 The anterior lobe is worn down nearly to the level of the ridge ; the surface describes a 

 transverse irregular ellipse ; that of the posterior lobe is narrower : in both a mid linear 

 tract of ostco-dentine (o, o) is exposed. The narrow hind basal ridge {g) is continued 

 upon the hind lobe as in j9 4, and that lobe projects clear beyond the level of the 

 grinding-surface of 7n 2. The antero-posterior and transverse diameters of the working- 

 surface of m 1 arc each 1 inch 6 lines. 



The anterior basal ridge (/) is strongly developed in m 2, and the antero-posterior 

 diameter of the tooth (1 inch 9 lines) rather exceeds the transverse diameter. The front 

 loljc {a) is worn down to within 5 lines of the basal ridge. The minor degree of abrasion 

 of the hind lobe shows the curve of the grinding-surface, concave backward, which is lost 

 as the thicker part of tlie lobe is reached. The hind basal ridge {g) is feebly developed. 



In the last molar {m 3) the hind lobe is markedly less than the front one, by its more 

 rapid loss of transverse dimension : it is rather narrower in this hne at its base, as it is 

 in fore-and-aft extent. The last upper molar of Diprotodon may be readily determined 

 by its i)ostcrior contraction. In some individuals the hind surface of the hind lobe 



