THE SKKLF.TON OF THE VERTEIiRATE HEAD. 133 



form, along with theii- cartilagiaou.s SL'ijtum or centrum, a 

 complete cataceutric neural ring. The interposition of the 

 ento-pterygoid of the cheloniau separates the meta-neura- 

 pophyses from the neurapophyses of the ethmoidal sclerotome, 

 and at the same time sepamtes the neural space into an upj^er 

 portion, mesially divided hy the cartilaginous septum or 

 centrum for the passage of the olfactory neiTes, and into an 

 inferior, mesially divided by the ento-pteiygoid itself for the 

 right and left nasal passages. A similar but somewhat 

 modified change is effected in tkc ethmoidal sclerotome of the 

 crocodilian, by the interposition of the anterior extremities of 

 its pterygoids — which anterior processes I believe to be, in 

 fact, the ento-pter}^goids. These anterior, generally mesially 

 united, processes of the pterygoids of the crocodilian, were 

 considered by Cuvier as representing the under portion of the 

 mammalian vomer. He describes them as two osseous pieces 

 fixed to the inner margins of the " palate-bones," in front of 

 the " anterior frontals," and of that part of the pterygoid 

 which covers the nasal canals. Professor Owen describes 

 these pieces as the " vomer," and as being generally anchylosed 

 to the fore part of the basi-sphenoid ; but he adds the follow- 

 ing very important observation, which I have verified, that 

 they (the " vomer ") form a distinct bone in a species of alli- 

 gator, which passes so far forward and downwards as to 

 appear in the fonu of a plate in the vault of the palate, in 

 front of the palate-bones. 



That this double bony splint is not a vomer, as Cuvier 

 suppose"!, must be evident, if the vomer is to be considered as 

 an element of the vomerine sclerotome. It cannot be, as 

 Professor Owen states, a vomer miited to the " basi-s}>henoid ;" 

 because, in front of the elevated, laterally compressed, quad- 

 rilateral process which passes forwards and upwards from the 

 centrum of the })ost-sphenoid, the real axis of the skull is 

 continued forward in the form of a compressed cartilaginous 



