PEOFESSOE OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 



531 



is less evenly concave transversely ; I have seen it almost canaliculate. The loss of 

 breadth of this lobe is chiefly from the outer side, and the lobe is lower than the front 

 one, the level of the grinding-surface reaching halfway toward that of the front lobe. 

 The fore-and-aft extent of the base of the tooth is 2 inches ; the transverse extent of the 

 worn surface of the front lobe is 1 inch 6 lines ; that of the hind lobe is 1 inch 3 lines. 

 The anterior ridge (fig. 4, /) is continuous with a feeble rising of the enamel at the 

 outer and the inner borders of the front surface of the anterior lobe. The posterior 

 basal ridge (Plate XXXVIII. fig. 3, g) is more directly and conspicuously continued into 

 the ridge along the inner border of the posterior surface of m 3. 



Wherever sufiicient of the lobes remains, their profile, especially the outer one, de- 

 scribes a curve concave forward (Plates XXXVII., XXXVIII. fig. 1). The inner and 

 anterior angle of each tooth, due to the more prominent part of the front basal ridge, 

 projects inward, a few lines beyond the inner surface of the tooth in advance (ib. ib. 

 fig. 2). Thus there is not only a zigzag disposition in the vertical but in the transverse 

 arrangement of the upper molars, though in the latter it be but slightly marked. The 

 enamel is about a line in thickness, and shows strongly the reticulo-punctate or rugous 

 surface at the less exposed parts of the crown. 



The upper molars are implanted by fangs which acquire twice the length of the ena- 

 melled crown : they are at least three in number, save in the first small and early deci- 

 duous tooth [d 3). The base of the anterior division of the tooth bifurcates as it de- 

 scends, slightly contracting in the socket, and thus forming two fangs in the same trans- 

 verse line. The base of the posterior division, if it bifurcates in any molar, is divided 

 later and to a less extent. It gradually contracts, and is longitudinally excavated at the 

 side next the other fangs. 



Figure 5, in Plate XXXVIII., gives a view of the two anterior fangs [m, n) of the last 

 molar ; fig. 6, ib., shows the single posterior fang [1] of the same tooth. Plate XXXIX. 

 fig. 3 shows the sockets and implanted ends of the fangs of the antepenultimate and last 

 molar teeth. The outstanding antero-posteriorly compressed zygomatic process of the 

 maxillary {ii!) is here opposite the hind lobe oim 2. 



In the series of upper molars of Bijjrofodon there are varieties as to size, and as to 

 order or degree of wear, the former variety being more constant. Both are exemplified 

 in the specimens figured in Plate XXXVII. figs. 2 & 3. In fig. 3, a portion of the left 

 upper jaw with the last three grinders, m 1 shows both lobes and the anterior ridge worn 

 down to a common field of dentine (d) and osteo-dentine (o, o) : the summits of m 3 are 

 partially abraded. In fig. 2, in which the last molar [m 3) shows an equal degree of 

 abrasion, the antepenultimate molar {in 1) is not worn to the same degree as in fig. 3; 

 the anterior lobe is ground down near to the basal ridge {f), but this remains untouched ; 

 the valley between the two main lobes is not obliterated. What is still more unusual, 

 where the last molar has come into use, the second molar [p 4, fig. 2, Plate XXXVII.) 

 preserves its lobes hardly worn down to the bottom of the valley, and the two fangs of 

 the first molar [d 3) remain in their alveolus. 



