186 



ON THE STRUCTURE OF THE SKULL. 



on the other hand, the hyomandibular altogether belongs to the 

 second visceral arch, while the hinder crus of the palato-qnadrate 

 belongs to the first, but has become detached from its primitive 

 connection with the basis cranii. 



The basi-occipital originates as an ossification, which imme- 

 diately surrounds and incloses the end of the notochord, and 

 extends into the adjacent cartilage. The ex-occipital is de- 

 veloped within the substance of the cartilaginous cranium on 

 each side of the basi-occipital. The parasphenoid, on the other 

 hand, is developed as a superficial ossification in the perichon- 

 drium of the base of the skull, and extends in front of, and 

 behind, the pituitary fossa in this membrane. The pre-maxillae 

 and maxillae have no cartilaginous predecessors, nor have the 

 dentary and angular pieces of the lower jaw. The palatine is 

 developed around, if not in, the anterior crus of the palato- 



Fig. 73. 



AS. US Tr:() 



Fig. 7."). — The cartilaginous cranium of a Pike, with its intrinsic ossifications viewed, A, 

 from above; B, from below; C, from the left side. N, N, nasal fossa;; I. Or, 

 inter-orbital septum ; a, groove for a median ridge of the parasphenoid ; b, canal for 

 the orbital muscles. 



