272 



greater degree toward the symphysis. The vertical diameter at the back part of the 

 symphysis is 2 inches in Nototherium inerme ; in N. Mitchelli it is 2 inches 10 lines ; 

 yet the fore-and-aft extent of the four last alveoli is G 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 Victorice; 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 Diprotodon, does not communicate 

 with any canal leading to the outer surface of that ramus, as is the case in Phascolomys 

 and the Poephaga. 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 Mitchelli or in N. Victorice. 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 Iihizophagous families. 



D. Comparison of the Mandible. — In comparing the mandible of Nototherium with that 

 of Diprotodon, 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 Diprotodon, and it is 

 also, on an obvious mechanical principle, strengthened or rendered more massive by the 

 presence of the pair of subsymphysial tuberosities *, 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 Diprotodon f. 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 Diprotodon and a convexity in Nototherium. These differences are more 

 marked in the adult than in the young animals, becoming more conspicuous in 

 Diprotodon as the incisive tusk acquires its adult proportions. 



In all the Nototherian mandibles the lower border is inflected at two parts ; the one 



* Plate XIX. figs. 1 & 2, I, t, p. 197. 



t Plate XIX. fig. 1, t; and compare Plate XXVII. fig. 2, with Plate XIII. fig. 3. 



