SKULL OF THE CKOCODILE. 105 



the cerebellum ; it is traversed by tympanic air-cells, and 

 assists with the exoccipitals, 2, 2, in the formation of the 

 chamber for the internal ear. 



The chief modification of the occipital segment of the 

 skull, as compared with that of the osseous fish, or with 

 the typical vertebra, is the absence of an attached haemal 

 arch. We shall afterwards see that this arch is present 

 in the crocodile, although displaced backwards. 



Proceeding Avith the neural arches of the crocodile's 

 skull, if we dislocate the segment in advance of the occi- 

 put, we bring away, in connection with the long base- 

 bone, 5, the bone, 9, which in the figure of the section of 

 the serpent's skull (Cut 17) is shown similarly united to 

 5. In fact, the centrums of the vertebrae have here co- 

 alesced, as we find to happen in the neck of the siluroid 

 fishes, and in the sacrum of birds and mammals. The 

 two connate cranial centrums must be artificially divided, 

 in order to obtain the segments distinct to which they 

 belong. The hinder portion, 5, of the great base-bone, 

 which is the centrum of the parietal vertebra, is called 

 " basisphenoid." It supports that part of the " mesence- 

 phalon," which is formed by the lobe of the third ven- 

 tricle, and its upper surface is excavated for the pituitary 

 prolongation of that cavity. The basisphenoid develops 

 from its under surface a " hypapophysis," which is sutu- 

 rally united with the fore part of that of the basioccipital, 

 but extends further down, and is similarly united in 

 front to the " pterygoids," 24. These rough sutural sur- 

 faces of the long descending process of the basisphenoid 

 are very characteristic of that centrum, when detached, 

 in a fossil state. The neurapophyses of the parietal ver- 

 tebra, 6, 6, or the " alisphenoids," protect the sides of the 

 mesencephalon, and are notched at their anterior margin. 



