THE SKULL. 75 



interorbital septum perforated by the olfactory nerve is present, it 

 is arrested in the orbital region (compare p. 57). 



The parasphenoid, which plays so important a part as an 

 investing bone of the roof of the mouth in Fishes and Amphibians, 

 begins to disappear ; amongst Reptiles it only attains to any im- 

 portant development in Snakes, where the anterior part remains, 

 and forms the base of the interorbital region. Its place is taken 

 by two cartilage bones, the basioccipital and basisphenoid, situated 

 along the basis cranii. In contradistinction to the Amphibia, only 

 a single condyle connects the skull with the vertebral column; 

 this, on close examination, is seen to be formed of three parts 

 (basioccipital and exoccipitals). 



The roofing bones of the skull are well developed, as in 

 Teleostei, while the trabecular region (ali- and orbitosphenoids) 

 becomes of secondary importance, its place being taken by processes 

 growing downwards from the frontal and parietal, especially in 

 Snakes. The parietals are usually confluent in the adult, and in 

 Lacertilia are perforated by an aperture (parietal foramen). 



For the topographical relations of the several bones to one 

 another compare Figs. 61 to 64. It will be seen in them that the 

 ground-plan of the Urodele skull, already somewhat fully explained, 

 is here fundamentally retained. 



A new element, the transverse bone (Figs. 61, 62. and 64, Ts), 

 extending from the maxilla to the pterygoid, appears, except in 

 Chelonia and Typhlopidae. An imperfect circum orbital ring of 

 bones present in Lizards is also worthy of mention. The dentition 

 is stronger than in the forms as yet described, and may be borne, 

 as in Amphibians, on the palatines and pterygoids as well as on 

 the proper jaw-bones (Fig. 62, PI, Ft). Rasp-like sphenoidal 

 teeth are not present in Reptiles, and the Chelonia are altogether 

 toothless, the free edge of the jaws being covered by sharp horny 

 sheaths. 



The skull of Crocodiles is of particular interest, owing to the 

 fact that the palatine processes of the maxillas (Fig. 64, Jf),,as well 

 as the palatines and pterygoids further behind (PI and Pi), meet I 

 together in the middle line, and thus form a secondary roof to^ the \ 

 mouth-cavity, separate from the proper '(sphenoidal) base of {he \ 

 skull. The cavity thus formed closes in the posterior prolongation 

 of the nasal chambers, in consequence of which the latter become 

 sharply differentiated from the mouth, and open far back into the 

 pharynx (Fig, 64, Oh). Thus the skull reaches a higher stage of 

 development, which,, only indicated in Chelonia, is characteristic 



Mammals. In__all Reptiles the suspensorium consists mainly 



the quadrate, which may be loosely attached to the skull 

 'Snakes, 1 Lacertilia), or firmly fixed to it (Hatteria, Chelonia, 



1 In Snakes i^Fig. 62, Qu) (except Tortrix), the quadrate is only indirectly con- 

 nected with the skull by means of the sqiiamosal .(Squ}, which extends backwards, 

 and thus throws the articulation of the lower jaw far backwards, giving rise to a very 



