206 DEVELOPMENT OF THE MARSUPIAL SKDLL, 



ment there are a number of points of difference of a more 

 fundamental nature. 



The nasal capsule has the anterior nares relatively further 

 forward than in Trichosurus, so that, whereas in the latter the 

 premaxilla lies underneath the middle of the opening, in Dasy- 

 urus the whole of the opening is in front of the bone. There is 

 a further difference in that while Trichosurus has a very imperfect 

 cartilaginous floor, in Dasyurus the floor is well developed. The 

 paraseptal cartilages are large in front, and pass backwards to 

 near the end of the nasal capsule; but posteriorly they do not 

 fuse with the back part of the capsule as in Trichosurus. 

 Posteriorly the nasal cartilages fuse with the median cartilage 

 and form a broad, flat, cranial floor. 



At this stage the alisphenoids are, as in the second stage of 

 Trichosurus, fused to the cranial floor-cartilage. They are, how- 

 ever, clearly differentiated from the median cartilage. From the 

 anterior and outer corner there passes upwards, outwards, and 

 forwards, a slender cartilaginous process wjhich, at its upper end, 

 joins with the orbitosphenoid cartilage. Though in a number of 

 respects unlike the ascending process seen in Trichosurus, it is 

 doubtless homologous with it, as in each case the cartilage lies 

 between the second and third branches of nerve v. The carotid 

 foramen is relatively smaller in Dasyurus. 



The orbitosphenoid cartilage is very large, and is completely 

 fused with the nasal capsule in front and the auditory capsule 

 behind. Less distinctly it is continued into the supraoccipital. 



Owing to the imperfectly developed state of the cochlea at this 

 stage, the lower part of the auditory capsule is small, but the 

 upper part containing the semicircular canals is quite as large as 

 in TrichosiLrus. 



The occipital region is relatively short. There are two fora- 

 mina for nerve xii., and a large opening is left in front'for nerves 

 ix., X., and xi. The occipital cartilage meets the auditory, but 

 does not fuse with it, except above the level of the semicircular 

 canal. 



