PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
523 
meters. It opens directly backward, the plane of its outlet being vertical. On each side 
of the inlet of the foramen magnum a wide and deep impression of a sinus curves down- 
ward to the jugular foramen at the inner side of the base of the paroccipital. The inner 
openings of the precondyloid canal are in a slight depression on each side the foramen 
magnum, a little nearer together than the outlets. 
In the almost entire skull the upper border of the foramen magnum and contiguous 
part of the superoccipital surface are wanting. In the part of the occipital of another 
skull that surface is preserved to an extent of 2 ^ inches in advance of the upper border 
of the foramen, and for a breadth of 6 inches. This surface slopes forward from the 
foramen and condyles as in the entire skull ; it is externally smooth and transversely un- 
dulating, showing a shallow medial concavity between two broad gentle convexities, which 
fall outwardly into concavities bounded by the oblique bases of the condyles. Nearly 
the lower half of the superoccipital surface is preserved in the fragment : the upper half, 
present in the skull, shows a strong medial vertical ridge (Plate XXXV. fig. 3, 3), and is 
bounded above by the ridge between the superocciptal (3) and parietals ( 7 ), continued 
outwardly upon the mastoids (s). The cranial air-sinuses are continued backward into 
the super- and ex-occipitals, not into the basioccipital. On each side the mid-ridge (3) 
is a shorter vertical ridge. The mastoid (s) makes a much less projection than the par- 
occipital ( 4 ) ; it is confluent above with the petrosal, as in other Marsupials ; not pre- 
serving its primitive distinction from that sense-capsule as in the Babyroussa*. 
The brain being small in Marsupials, and the disproportionate smallness of its case to 
the rest of the skull increasing, as in other natural groups of mammals, with the general 
bulk of the species in such group, this character is a striking one in the skull of Dipro- 
todon. Like its carnivorous contemporary the Thylctcoleo f, the brain-case makes no 
t convexity or out-swelling into the temporal fossae ; the inner as well as the outer and hind 
walls of these long and large lateral vacuities are concave, and form parts of a general 
though not uniform excavation. 
The broad and low triangular superoccipital surface, strongly sloping forward as it 
rises from the condyles, contracts above to its apex, and is continuous there with a 
(sagittal) ridge of the coalesced parietals (ib. fig. 1, 7 ) extending forward to the inter- 
orbital region. There the upper surface of the cranium begins to expand, and to swell 
into a pair of low convexities (ib. fig. 1, n) which roof over the frontal sinuses. The 
outer wall of this pneumatic part of the cranium has been crushed down by posthumous 
pressure or injury in the entire skull. 
The nasals (Plate XXXV. figs. 1, 2, n ), in continuing forward from the frontals the 
upper line of the skull, rise gently toward their terminations, which again curve 
downward, giving a sigmoid contour to that part of the cranial profile in a degree peculiar 
to the present species. The vertical diameter of the facial part of the skull at the ter- 
* Owen, ‘ Anatomy of Vertebrates,’ 8vo, vol. ii. p. 469 ; and ‘ Catalogue of the Osteology, in the Museum of the 
Royal College of Surgeons,’ 4to, no. 3345, p. 557. 
t Philosophical Transactions, 1866, Plate i. 
