PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
269 
across anteriorly. There is a slight swelling of the base of the hind surface of the hind 
lobe, but not any distinct postbasal ridge. 
I have given a new figure of the side view of part of this fossil (Plate XXII. fig. 4)*, 
and an upper view of the entire fragment (ib. fig. 3), showing the characters of the 
working-surface of the .molars. 
In a visit this year to the Geological Museum, Oxford, I was much gratified and 
interested in finding, in the series of fossils from the freshwater deposits of Darling 
Downs presented by Sir Charles Nicholson, Bart., M.D., evidence of which I had been 
long in quest, of the fully, or nearly fully, developed dentition of the lower jaw of 
Sthenurus Atlas. Through Professor Phillips’s kind permission, this unique fossil forms 
the subject of figs. 5-8, Plate XXII. It is a left mandibular ramus, wanting the ascend- 
ing branch, of a nearly mature individual of Sthenurus Atlas. 
The last molar (m 3) has risen into place, and the summits of its transverse lobes have 
been just touched by masticatory abrasion, acting from above obliquely backward, 
without exposing the dentine ; but the large and characteristic premolar (p 3) has not 
risen beyond the level of the basal third of the crown of the adjoining molar ( d 4), and 
its summit is quite unworn. 
This specimen, moreover, gives the mandibular characters of the genus Sthenurus as 
distinguished from those of Macropus (ib. figs. 13, 15) — as, e.g ., the shorter sym- 
physis (fig. 6, s), the larger extent thereon of the articular surface (which reaches to the 
outlet of the incisor socket), the angle which its lower border makes with that of the 
horizontal ramus, and the continuation of the upper or diastemal border to the 
incisor outlet in a direction more nearly parallel with that of the molar alveolar border, 
not descending so much or so abruptly from that border as it advances forward. 
The outlet (ib. fig. 5, v) of the dental canal is nearer the molar series, and the part of 
the jaw anterior to the outlet is shorter than in Macropus Titan. The depth of the 
ramus behind the last molar (m 3) is relatively greater. The inner surface of the hori 
zontal ramus (ib. fig. 6) is less convex vertically than in Macropus Titan. 
The symphysial surface, though free or unanchylosed in the not quite mature indi- 
vidual yielding the specimen, must, from its greater vertical extent and uniform flat- 
ness, fit closer to its fellow, and permit less divaricating movements of the two rami 
than in Macropus. Besides the anterior outlet (ib. fig. 5, v) there is a vascular foramen 
below m 1 , midway between the alveolar and inferior borders of the ramus ; but this 
may be an individual character. 
The broken border of the ascending ramus shows the fore half of. the margin of the 
wide intercommunicating foramen (ib. fig. 6, e), and the fore part of the large cavity 
from the inner half of which the dental canal is continued forward. 
The postalveolar platform has a sharper inner border, and forms a more marked 
angle at that border, than in Macropus, indicating the place of the postalveolar process 
in Nototherium, to which, in the form and proportions of the symphysis, its closer and 
* It is shown entire from this view in “ Mitchell,” op. cit. 1st ed. vol. ii. pi. xxix. fig. 1 . 
MDCCCLXXIV. 2 N 
