PEOFESSOE OWEN ON THE FOSSIL MA^IMALS OF AUSTEALIA. 



315 



In the marsupial Carnivora 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 imderlapped 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 likewise in the marsupials, but, as they extend backwards to join the 

 alisphenoidal bullae, they diverge from the basisphenoid ridges and are external to them. 



Sufficient 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 

 ridges (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 

 alisphenoid 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-posterior extent of the former, and the degree to which they underlap the 

 basisphenoid, leaving only a strip 2^ lines broad exposed at its junction with the presphe- 

 noid, 9 ; and gradually diverging as they extend backward, the basisphenoid, s, 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 

 for in vain in any mammalian Carnivora, save those of the marsupial order. 



In the placental Carnivora, the superoccipital region, defined below by a boundary 

 line dravm across the upper ends of the condyles, is almost as high as it is broad, and 

 in rising from the foramen magnum it curves slightly backward. In the marsupial 

 Carnivora 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 

 in the Thylacoleo; the occiput being relatively as broad as in Sarcophilus, and the 

 superoccipital sloping more forwards than in the Thylacine before it rises vertically to 

 the 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 

 family 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 

 Carnivora, and the lateral walls of the cranial cavity make a smaller protuberance or 

 convexity at the temporal fossae. In the remarkable skull under comparison, the sides 

 of the cranial carity make no protuberance whatever into the temporal fossae ; they have 



