PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
45 
The base of the huge zygomatic arch is continued (Plate II. fig. 1, 27), with a slight 
sinking inward, from the whole vertical extent of the mastoid ridge and from a part of 
the superoccipital ; the lower end being formed by the tympanic, which is defined by a 
slight notch from the end of the mastoid process. 
The parietal walls (ib. 7) extend from without inward and forward. From the short 
alisphenoid the parietal plate arches upward, with a strong convexity forward at its 
lower half (Plate III. fig. 3, 7 ) ; this subvertical part of the cranial walls forms the 
hind boundary of the vast subquadrate oblong vacuity combining orbit and temporal 
fossa (Plate III. figs. 2 and 3, t). The parietal or parieto-temporal wall (Plate II. fig. 1, 7 ) 
is divided from the occipital plane (Plate III. fig. 2, 3 ) by the superior or superoccipital 
arched ridge ; it is divided from its fellow or opposite wall above by a flattened tract 
about an inch broad (ib. fig. 2, 7 ), near the superoccipital (ib. fig. 2, 3 ), but which expands 
as it advances from the parietal (7) upon the frontal (n) region. The parieto-frontal part 
of the cranium forms less than the middle third of the breadth of the entire skull as 
here completed by the enormous zygomatic arches. The frontal roof of the cranium, 
retaining its flatness transversely, gains a breadth of five inches, with a slight downward 
slope in profile (Plate II. fig. 1), and then (ib. n) more abruptly arches down to the 
origin of the nasals (ib. 15), an arch being continued outward, on each side of the naso- 
maxillary pedicle, to the tuberosity (s) representing the antorbital or lacrymal process. 
There is a transverse depression above the origin of the nasal bone (Plate III. fig. 2, is). 
The vertically convex outswellings of the frontal above and alongside this depression 
indicate the enormous air-sinuses within. The inner side or walls of the orbito-temporal 
vacuities sink sheer from the upper parieto-frontal tract to the outswelling of the maxil- 
lary molar alveoli (ib. 21 ), with a slight inclination inward. The greatest posterior depth 
of this cranial precipice is 6| inches. 
At the junction of the alisphenoid with the parietal, near the bottom of the back 
wall, is a tuberosity. The diameter of the sphenoido-parietal part of the cranium is 4^ 
inches; that of the skull at the corresponding part across, or including the zygomatic 
arches, is 16 inches! The cranium proper, from this singular constriction, gradually 
expands as it advances to the superorbital part of the frontals. If the cranial cavity 
concurred with its outer walls in shape it would be triradiate, two corridors extending 
along the transversely extended and antero-posteriorly contracted occipital part, and a 
third passage running forward from the mid line toward the face. But the singular 
departure in the outer walls from the normal shape of the brain-case is mainly due to a 
vast diploe of air-cells. The proper cerebral cavity makes no outward show, and it is 
insignificantly small in proportion to the entire skull. 
Viewed from below (as in Plate III. fig. 3), the condyles ( 2 , 2 ) are divided by a deep 
notch; their lower ends descend a little below the level of the basioccipital ( 1 ). This 
presents a rugged triangular tract in advance of the foramen, the apex being continuous 
with a sharp ridge longitudinally bisecting the surface of the basisphenoid. On each 
side of the tuberous tract and ridge is a wide and moderately deep depression, extending 
