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

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

 outside the last molar (Plate XIX. 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 (f, f). The alveolar border is continued into an obtuse ridge or 

 prominence (Plate XXVII. fig. 2, p), 2 inches behind the last alveolus ; from which 

 prominence the ridge subsides and expands, retrograding to form the internal border 

 of the entry of the dental canal (ib. ib. o). 



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

 molar socket (Plate XIX. fig. 1, 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. sa). Below this 

 orifice the ramus bulges out into a rather rough tumefaction, then slopes and contracts 

 upward and forward to complete the socket of the huge procumbent lower incisor (i). 

 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 XXV. 

 fig. 2, s, s J ). 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 Hippopotamus (Plate XIX. fig. 2, t, t). 



The symphysis (Plates XXV., XXVI. 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 XXV. fig. 2, s, s 

 with Plate XXVI. 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 which 

 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 

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

 forward ; 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 Diprotodon with that of any other 

 large quadruped carrying a single incisor in each ramus there are well-marked differences. 

 The symphysis in the Sumatran Rhinoceros and in Acerotherium is less deep and is pro- 

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

 its deflection in Dinotherium more conspicuously differentiate them. In the rernark- 



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