PEOFESSOE OWEN ON THE FOSSIL MAMMALS OF ATJSTEALIA. 
59 
last formative alveolus, and it contracts as it inclines toward the outer wall of the ramus 
in its forward course. 
The contour of the lower border of the ramus from the hind fracture to the sym- 
physis (figs. 1 & 4, e, s') is a more open curve than in the adult; it is feebly interrupted 
between the inflected border ( d ) and the hinder inflection or angle a ; the slight con- 
cavity between d' and a being less apparent in the adult jaw. The ridges (fig. 1, li, q) 
bounding the ectocrotaphyte depression (f) are naturally feebler, less pronounced, in this 
young jaw; the base of the anterior one (q) rises from the transverse parallel of the 
hind lobe of the penultimate molar (m 2 ). The postinternal angle of the formative 
alveolus of the last molar appears to represent the postalveolar process of the mature 
mandible. 
The oblique channel (answering to u in figures of the adult jaw) between the coronoid 
and postmolar processes here runs from that lodging the fore lobe of the penultimate 
molar to near the middle of the outer part of the interspace between the lobes of the 
antepenultimate molar (m 1 ) ; it thus preserves its general relative position to the last 
grinder “ in place” and use, and doubtless was still more advanced when m 1 was “ en 
germe.” 
Such changes in the relative position of parts, and differences of general shape, of the 
mandible in the adult and young Noto there are dependent on, or concomitant with, 
the growth called for to sustain in action the full complement of teeth in the adult. 
No inference of specific difference can be deduced from the relative position of the hind 
part of the “ symphysis mandibulae” (i) in this young jaw to the front lobe of the second 
molar ( d 4) ; because the socket of that tooth would move forward in the course of growth, 
whilst the symphysis extended its grasp of the fore parts of the two rami prior to the 
ultimate obliteration of the syndesmotic joint in the adult. At the present immature 
stage this articulation remains. The surface (fig. 4, s*) is vertical, flat, with roughish 
rugae, mostly directed from above downward and forward, gaining in prominence, through 
deepening of the intervals, along the lower third. It seems as if confluence had already 
begun at a small part of the upper and posterior border of the articular surface, such 
portion having been broken away from the left ramus and left adherent and seemingly 
confluent with the right one. Behind the lower part of the posterior border of the 
symphysis is the flattened, rough, slightly depressed surface (fig. 2, v) for muscular 
insertion noted in the older specimens. 
The shallow indent or concavity dividing the inflected parts of the horizontal ( d ') and 
ascending ( a ) rami has a more advanced position and a direction more approaching 
the horizontal than in the mature jaw: in Plate VI. fig. 4, a-d! is shown to be below 
the interval between the penultimate and last molars and parallel in extent with the 
contiguous lobes of these teeth. The inward extension of the bone at a, fig. 4, represents 
a resumption of the inflection of the lower margin of the jaw at its hinder part, from 
which resumption the bone thins off to be continued backward into the thickened part ( e ), 
which contributed to support the broken-off condyle. 
1 2 
