OAYEN NOTOTHERITJM. 177 



Hughes, from freshwater (pleistocene ?) deposits of Darling Downs, 

 contains the right ramus of the mandible of the Nototherium 

 inerme (PI. IX. fig. 3*), very closely corresponding with that figured 

 in pi. 3 of my " Report" and " Catalogue." It fortunately includes 

 sufficient of the symphysis to show the bottom of a socket of a 

 small procumbent incisor. One of the diiferences between the 

 cranium of the great Lvprotodon and that of the smaller animal 

 with double-ridged molars subsequently acquired is the relatively 

 smaller size of the incisors in the so-called Zygomaturus. From 

 the analogy of the Diprotodon and of its existing representatives, 

 Macropus and Phascolarctos, the six upper incisors of Zygo- 

 maturus would be opposed by a single pair at the fore part of the 

 lower jaw. 



But this pair would be so much smaller in Nototherium than in 

 Diprotodon as to leave no trace of their sockets in that part of 

 the jaw — ^viz. beneath the two anterior molars — ^where the corre- 

 sponding socket is widely excavated in Diprotodon : the diiference in 

 the size and position of the incisor- socket was, in fact, such as led 

 me to infer that Nototherium did not possess a tooth developed to the 

 degree which is indicated by the term " tusk"t ; and the fossil jaw 

 transmitted by Mr. Hughes proves such to be the case, and that the 

 inferior incisor presented the same small proportional size, com- 

 pared with Diprotodon, which the upper incisors of the so-called 

 Zygomaturus present. Precisely the same characters which dis- 

 tinguish generically the lower molar teeth of Nototherium from 

 those of Diprotodon distinguish the upper molars of Zygomaturus 

 from those of Diprotodon. This concordance is carried out even to 

 the minute markings of the enamel. 



With respect to that character in the lower molars of Nototherium 

 Mitehelli, I have remarked, " The dentine of the crown is encased in 

 a sheath of enamel of nearly one line in thickness, with a smooth 

 and polished surface, impressed at the outer part and near the base 

 of the tooth, where the enamel is principally preserved, with fine 

 parallel and nearly horizontal transverse lines J." Precisely the 

 same character is presented by the enamel of the upper molars 

 of Zygomaturus. I then proceeded to state, '' The smooth and 

 polished exterior of the enamel covering the anterior part of the 

 posterior eminence presents a striking contrast with the reticulo- 

 punctate character of the enamel at the corresponding part of the 

 molar in the Diprotodon §.^^ The upper molars of Zygomaturus differ 

 in the same way from those of Diprotodon. 



Besides the well-executed casts of the cranium, and of part 



* Drawn on the plate without reversal. 



t " The anterior end of the symphysis (fig. 4) is broken away ; but there is no 

 trace there of the socket of any tooth ; and it is too contracted to have supported 

 any tusk or defensive incisor." (Extinct Mammals of Australia, 8vo, p. 12, 

 1845.) I erred, however, in supposing that incisors were absolutely wanting in 

 the lower jaw of the smaller species, thence called NototheriuTn inerme: the 

 analogy of the Rhinoceroses, however, supported the supposition that this species 

 might diifer from the larger Nototherium Mitehelli in the absence of those teeth. 



+ Bnd. p. 1.3. § Ibid. 



VOL. XY. PART I, N 



