256 ON THE STRUCTURE OF THE SKULL. 



tyrnpano-periotic fissure is closed, either by the close apposition, 

 or by the actual anchylosis, of the inner lip of the tympanic to 

 the periotic. 



But, in the Sheep and Pig, this fissure is replaced by a wide 

 elongated aperture, the inner edge of the tympanic bulla being 

 rolled in like a scroll. In the Seals and Cetacea the scroll-like 

 form of the immensely thick tympanic bulla becomes still more 

 marked, and the tympano-periotic fissure wider ; while the latter 

 is converted into a great gap in the floor of the tympanum in 

 Orycteropus and in the Sirenia, tie tympanic being reduced to 

 a mere thick hoop. 



In many Marswpialia the alisphenoid dilates posteriorly and 

 inferiorly into a funnel-shaped, thin-walled, bony chamber, 

 whicli closes the tympanic cavity anteriorly, uniting by its edges 

 with the tympanic bone. In certain Insectivora, such as the 

 Hedgehog and Tenrec, the tympanic cavity is partly walled in 

 by a process of the basi-sphenoid. 



In Hyrax, and in many Marswpialia and Rodents, the jugal 

 enters into the composition of the glenoid facet for the lower 

 jaw. In the Marsupials the alisphenoid may also contribute 

 towards the formation of this articular surface. In almost all 

 Marsupials the angle of the mandible is continued inwards into 

 a horizontal plate of bone. This " inflexion of the angle of the 

 jaw" is peculiar to these Mammals. 



The palatine and pterygoid bones present very considerable 

 differences in their connections among Mammalia. 



Thus, in the Ornithorhynchus, in the larger Myrmecophagae, 

 and in some Cetacea, the pterygoids unite in the middle line 

 below, so as to prolong tne bony palate beyond the palatines, as 

 in the Crocodiles. In the Marsupials, on the other hand, the 

 bony palate, formed only by the maxilla? and palatines, is often 

 defectively ossified, so that large open spaces are left therein 

 on the dry skull. 



In order to understand the changes which the normal type 

 of skull undergoes in the Mammalian series, it is necessary to 

 define a few lines and planes by the help of certain well-marked 

 organic fixed points. 



