342 



ANATOMY OF VERTEBRATES. 



Phascolarctos. 



The glenoid cavity presents a cliaractenstic structure in most 

 of the Marsupialia in not being exclusively formed by the squa- 

 mosal. With the exception of the Petaurists, the malar bone 

 forms the outer part of the articular surface for the lower jaw, 

 and in the Thylacinus, Dasyurus Maugei, Dasyurus ursinus, 

 Perameles, Hyj)si])rymniis, and Macropus, the alisphenoid forms 

 the inner boundary of the same surface. In the Kangaroo, 

 Dasyures, Koala, and Wombat, the alisphenoid articulates mth 

 the parietal, but by a very small portion in the two latter species : 



in the Perameles and Po- 



221 



toroos the alisphenoid does 

 not reach the parietal. 



In \hQ parietals,fLg. 221, 

 7, the sagittal suture is ob- 

 literated in those species 

 in which a bony crista is 

 developed in the corre- 

 sponding place. They pre- 

 sent a singularly flattened 

 form in the Wombat, in an 

 aged skull of which, and 

 in a similar one in the 

 Kangaroo, I observe a like obliteration of the suture. In the 

 Kangaroo, Potoroo, Petaurus, Phalanger, and Myrmecobius, 

 there is a triangular interparietal bone. The corresponding bone 

 I find in three pieces in the skull of a Wombat. 



The frontals, ib. ii, are chiefly remarkable for their anterior 

 expansion and the great share which they take in the formation 

 of the nasal cavity. In the Thylacine the part of the cranium 

 occupied by the frontal sinuses exceeds in breadth the cerebral 

 cavity, from which it is divided by a constriction. The coronal 

 suture presents in most of the Marsupials an irregular angular 

 course, forming a notch in the frontals on each side which receives 

 a corresponding triangular process of each parietal bone. A 

 process corresponding to the posterior frontal augments the bony 

 boundary of the orbit in the Thylacine, the Ursine Dasyure, and 

 in a slighter degree in the Virginian Opossum ; it is relatively 

 most developed in the skull of the Myrmecobius fasciatus, where 

 the orbit is large; but the bony boundary of the orbit is not 

 complete in any Marsupial. In the Myrmecobius there is a deep 

 notch at the middle of the superorbital ridge. A corresponding 

 but shallower notch is present in the skull of Petaurus sciureus. 

 I have found the frontal suture obliterated in old specimens of 



