PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
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 (f). 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. cl). 
The horizontal ramus gains slightly in depth as it advances from the last to the first 
molar socket ( cl 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 (?'). From the socket of cl 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 Hip- 
popotamus (Plate XXXV. fig. 2, t, t ). 
The symphysis (Plates XLI., XLXI. 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 XLI. 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 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 
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 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 Acer other him 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 remark- 
mdccclxx. 4 c 
