PEOFESSOE OWEN ON THE EOSSIL MAMMALS OF AUSTEALIA. 



527 



The anterior border of the ascending ramus is straight and subvertical ; it is thickened 

 at its lower part to be continued into the convex outswelling of the horizontal ramus 

 outside the last molar (Plate XXXV. fig. 1, m 3), a distance of an inch intervening be- 

 tween the alveolus of this molar and the convexity rising and thinning into the fore part 

 of the coronoid plate (/). The alveolar border is continued into an obtuse ridge or 

 prominence, 2 inches behind the last alveolus ; from which prominence the ridge sub- 

 sides and expands, retrograding to form the internal border of the entry of the dental 

 canal (ib. d). 



The horizontal ramus gains slightly in depth as it advances from the last to the first 

 molar socket (d 3). Two and a half inches below this socket, and a little in advance, is 

 the vertically elliptic outlet of the dental canal (ib. 32). Below this orifice the ramus 

 bulges out into a rather rough tumefaction, then slopes and contracts upward and forward 

 to form the socket of the huge procumbent lower incisor (t). From the socket of d 3 the 

 alveolar border sinks and expands into the upper part of the socket of the incisor. The 

 under border of the horizontal ramus is smoothly and broadly convex transversely. The 

 inner surface sinks sheer from the openings of the molar alveoli, and curves inward 

 below the anterior ones to the symphysis (Plate XLI. fig. 2, s, s). The fore part of 

 the mandible below the incisive alveoli, expanding to the tuberous outswellings above- 

 mentioned, has a broad, subquadrate form, recalling the shape of that part in the Ilij)- 

 ])opotamus (Plate XXXV. fig. 2, t, t). 



The symphysis (Plates XLL, XLII. fig. 2, s, s)begins behind, at a line dropped verti- 

 cally from the front lobe of the third molar (m 1) ; it is 6 inches in length, 4 inches in 

 depth in the full-grown animal. It gains in vertical direction more than in length during 

 the growth of the mandible, with reference apparently to the provision of a sufficient 

 lodgment of the progressively increasing incisive tusk. (Compare Plate XLT. fig. 2, s, s 

 with Plate XLII. fig. 2, s, s.) 



The large size of the dental canal exposed by the posterior fracture of the ramus of 

 another mutilated mandible indicates the ample supply of vessels and nerves Avhich 

 minister to the growth and nutrition of the incisive tusk ; the depth of the symphysis 

 of the jaw corresponds with the tusks, which it helps to support; contributing to the 

 required strength for the operations of those eroding implements, with space for the 

 deep implantation and for the lodgment of the large persistent matrix of each tusk (Plate 

 XLII. fig. 5). The direction of the symphysis is oblique, from below upward and for- 

 ward ; its upper margin is nearly straight, its lower one convex ; the rough articular 

 surface stands out a very little way from the vertical plane of the inner surface of the 

 ramus. 



In comparing the symphysial part of the jaw of Diprofodon with that of any other 

 large quadruped carrying a single incisor in each ramus there are Avell-marked diflferences. 

 The symphysis in the Sumatran Bhinoceros and in Acerotlierium is less deep and is pro- 

 portionately broader ; the great length of that part in the Mastodon longirostris, and 

 its deflection in DinotlieHum more conspicuously differentiate them. In the remark- 



MDCCCLXX. 4 c 



