PEOFESSOE OWEN ON THE FOSSIL MAMMALS OF AUSTEALIA. 315 
In the marsupial Carnimra the basisphenoid is relatively longer than in the placental 
Carnivora, and, at its posterior part, it sends a ridge downwards from that part of each 
lateral margin which is not underlapped or covered by the base of the alisphenoid, the 
suture of which long continues distinct. These ridges, with the alisphenoid, render the 
whole under surface of the basisphenoid canaliculate, or concave transversely : the basi- 
sphenoid is flat beneath in the placental Carnivora, and that part of the base of the skull 
is made canaliculate by the development of the ectopterygoid plate from the alisphenoid : 
these plates exist likemse in the marsupials, but, as they extend backwards to join the 
alisphenoidal bullee, they diverge from the basisphenoid ridges and are external to them. 
Sufiicient of the base of our fossil skull remains to demonstrate this characteristic 
marsupial structure : the basisphenoid, though convex lengthwise beneath, is concave 
transversely by the production from the lateral margins of its hinder part of the same 
ndges (r, r) as those of the Thylacine and Dasyures, and in the degree of concavity more 
resembles the latter : the commencement of the outer ectopterygoid ridge (s) of the 
ahsphenoid is preserved, diverging as it extends backwards from its anterior junction with 
the basisphenoid ridge. 
The sutures between the alisphenoids and basisphenoid still remain, indicating the 
great antero-postenor extent of the former, and the degree to which they underlap the 
basisphenoid, leawng only a strip 2i lines broad exposed at its junction with the presphe- 
noid, 9 ; and gradually diverging as they extend backward, the basisphenoid, 5, being- 
one inch and a half in breadth at their hinder borders. 
The characters of the base of the cranium here displayed by the Thylacoleo, and the 
greater retention of the typical elementary construction of the skull, would be sought 
tor in vain m any mammalian Carnivora, save those of the marsupial order. 
In the placental Carnivora, the superoccipital region, defined below by a boundary 
line drawn across the upper ends of tlie condyles, is almost as high as it is broad, and 
m rising from the foramen magnum it curves slightly backward. In the marsupial 
armvora the same region so defined is much broader than it is high, especially in 
the Dasyures (Plate XV. fig. 2); in these the occiput is vertical; it inclines a little 
forward from the foramen magnum in the Thylacine. All these characters are repeated 
m the Thylacoleo-, the occiput being relatively as broad as in SarcopMlus, and the 
superoccipital sloping more fomards than in the Thylacine before it rises vertically to 
de occipital crest; thus departing in a greater degree from the placental type, and 
manifesting, as might be expected from the superior general size of the skull, in a more 
marked manner, the inferiority of development of the brain. In every natural group or 
amily of the warm-blooded Vertebrata the brain is proportionally less as the animal is 
larger, and its osseous case makes a smaller part of the entire skull. 
In the marsupial Carnivora the brain is relatively much smaller than in the placental 
amaom, and the lateral waUs of the cranial cavity make a smaller protuberance or 
convexity at the temporal fosste. In the remarkable skull under comparison, the sides 
0 e cranial cavity make no protuberance whatever into the temporal fossa: ; they have 
