PEOFESSOK 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 upj)er 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 (?), 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 Bipro- 

 todon. Like its carnivorous contemporary the Thylacoleo^, the brain-case makes no 

 convexity or out-swelling into the temporal fossse ; 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 coiidyles, contracts above to its apex, and is continuous there with u 

 (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, 15), 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. 



