262 



concavity to the outlets of the sockets ; the convexity subsides as the jaw advances and 

 the surface ascends more vertically to the outlets of the three anterior molars (ib. d 3, d *, 

 m 1 ), but it continues the vertically convex curve to the lower border. The thickness 

 of the ramus before inbending to the symphysis is 2 inches ; its height where it joins its 

 fellow at s (Plate XXXVIII. fig. 2) is 3 inches 5 lines. At the lower and back part of 

 the symphysis is a transverse roughish crcscentic depression (ib. fig. 3, v) for muscular 

 insertion. The general longitudinal lay of the outer surface of the horizontal ramus 

 is a feeble convexity forwards as far as below the second molar (d' 4), where it begins 

 to change to a concavity leading on to the symphysial part (fig. 1, 32-k), containing, 

 anteriorly, the sockets of the incisors. On the vertical parallel of the fore part of the 

 first molar socket, about halfway between the upper and lower borders of that part of the 

 ramus, is an outlet of the dental canal (ib. 32) ; it is subcircular, 5 lines in long diameter. 



The inner surface of the horizontal ramus (Plate XXXVIII. figs. 1 & 2, i) sinks sheer 

 below the outlets of the last socket, and with a slight vertical convexity from that of the 

 penultimate molar ; it is at first feebly concave, then convex to the back part of the sym- 

 physis, and the surface is uniformly concave at the upper part of the symphysis (ib. fig. 

 2, s*), between the three anterior sockets of the right and left sides. The longitudinal 

 lay of the inner wall of the ramus is feebly convex posteriorly, changing to a concavity 

 deepening into the back and upper part of the symphysis. This junction of the right 

 and left rami is completely ossified without a trace of the primitive separation shown in 

 Plate XL. figs. 2, 3, 4, s, s', s* ; herein contrasting strongly with the condition of the 

 joint in the Kangaroof . 



The hind surface of the symphysis (Plate XXXVIII. figs. 2 & 3, s), vertically convex 

 and smooth, is on the vertical parallel with the back lobe of third molar (m 1), near, but not 

 quite extending, to the interspace between its socket and that of the fourth molar (m 2). 

 The upper surface of the symphysis (ib. fig. 2, s*, s) between the three anterior molars 

 (m 1, d 4, d 3) is a rather deep smooth longitudinal canal, the margins of which begin to 

 be encroached on by a diastemal ridge (ib. k), continued forward from the socket of d 3 

 with a slight curve convex inward. 



The antero-posterior extent of the five molar alveoli is 7 inches 5 lines. The breadth 

 of the anterior division of the first socket is 3^ lines, of the posterior division 5 lines ; 

 the depth is shown in the jaw of the young Nototherium (Plate XL. fig. 5, I). The 

 sockets of the other molars increase in breadth to the anterior division of the last, which 

 is 1 inch 1 line across. The alveolar plate rises in an angular form at the intervals of 

 the sockets, and at those of the diverging roots of each tooth on both outer and inner 

 sides of the jaw. 



At the fractured part of the symphysis are parts of the bottoms of a pair of incisive 

 alveoli ; that on the left side gives a transverse breadth of 9 lines and a vertical one of 

 about 1 inch ; but the lower wall is broken away from the base. A still smaller por- 

 tion is preserved on the right side. 



t Owex, Osteology of the Marsupialia, ' Anatomy of Vertebrates,' vol. ii. p. 350. 



