64 
PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
whence the specific name originally suggested by the present fossil*. The depth of the 
horizontal ramus is relatively less than in Nototherium Mitclielli, and diminishes in a 
greater degree toward the symphysis. The vertical diameter at the back part of the 
symphysis is 2 inches in Nototherium inerme ; in N. Mitclielli it is 2 inches 10 lines ; 
yet the fore-and-aft extent of the four last alveoli is 6 inches in the former and 5 inches 
7 lines in the latter, the same specimens which afford the difference of depth of ramus 
yielding the latter admeasurement. 
The longitudinal extent in which the lower border of the ramus is inflected (fig. 3, d , d') 
equals that in Nototherium Victorias ; it is also interrupted at a similar part, but appa- 
rently less abruptly. The dental canal (fig. 3, o) perforates the smooth ridge or longi- 
tudinal rising of bone leading from the postmolar process toward the back part of the 
rising ramus, and, as in other Nototheria and in the Piprotodon, does not communicate 
with any canal leading to the outer surface of that ramus, as is the case in Phascolomys 
and the Po'epliaga. The anterior outlet of the dental canal is below the position for the 
socket of the first molar (d 3), which socket would seem to be obliterated and the tooth 
shed earlier than in Nototherium Mitclielli or in N. Victorias. In the forward slope of so 
much as is preserved of the posterior margin of the ascending ramus and its uninter 
rupted continuation with the convex curvature leading to the symphysis, in the presence 
and position of the postmolar process, in the position of the base of the coronoid process 
exterior to the hindmost molar, in the thickness of the horizontal ramus and the con- 
vexity of its outer surface, the present jaw exemplifies its resemblance to that in Phasco- 
lomys ; but it differs in the absence of the deep excavation on the outside of the ascending 
ramus, and in the inferior depth of the inner concavity due to the inferior extent of the 
inward production of the angle of the jaw, which marsupial character reaches its maximum 
in the smaller existing Poephagous and Rhizophagous families. 
D. Comparison of the Mandible. — In comparing the mandible of Nototherium with that 
of Piprotodon, the chief difference relates, as might be surmised, to the chief dental one, 
viz. to the development, in the larger marsupial Herbivore, of the mandibular incisors 
into deeply implanted scalpriform tusks. The part of the jaw supporting and wielding 
these instruments is accordingly both deepened and widened in Piprotodon , and it is 
also, on an obvious mechanical principle, strengthened or rendered more massive by the 
presence of the pair of subsymphysial tuberosities f , of which there is no trace in Noto- 
therium. The horizontal ramus in the smaller extinct genus is less deep in proportion 
to its breadth or thickness, and it loses depth at the symphysis instead of gaining it 
there, as in Piprotodon J. Consequently the lower contour of the horizontal ramus 
presents opposite curves in the two genera ; it passes to the symphysis, describing a con- 
cavity in Piprotodon and a convexity in Nototherium. These differences are more 
* ‘ Catalogue of the Fossil Mammalia &c. in Mus. Coll. Surg.,’ 1845, p. 314. 
t Philosophical Transactions, “ On the Fossil Mammals of Australia,” Part III. tom. cit. PI. xxxv. 
fig- 2, /, /. 
t Ibid. PL xxxv. fig. 1, s ; and compare PI. xlii. fig. 2, with Plate VIII. fig. 3 of the present Paper. 
