THE SKELETON OF THE VERTEBRATE HEAD. 177 



backwards, the so-called " pre-frontal," the " cranial ethmoid," 

 the " orbito-sphenoid " of Owen, the " ali-sphenoid " of Owen, 

 and the lateral occipital. I have already stated the grounds 

 on which I believe we must look upon the " pre-frontals " of 

 the fish as the neurapophyses of the ethmoidal, and the 

 " cranial ethmoid," as the combined neurapophyses of the pre- 

 sphenoidal neural arches. If so, then the succeeding plate 

 must be the " ali-sphenoid," and not the " orbito-sphenoid/' as 

 Professor Owen considers it to be ; and therefore, as there 

 has never been a question regarding the lateral occipital, the 

 plate interposed between the latter and the former, as it has 

 all the characters of a neurapophysis, indicates the existence 

 of a cranial segment between the post-sphenoidal and oc- 

 cipital. I shall not at present allude to the various opinions 

 entertained regarding this plate, but shall merely distinguish 

 it as the inferior temporal neurapophysis. 



Proceeding now to the consideration of the centrums cor- 

 responding to this series of neurapophyses, it must be ob- 

 served that in no osseous fish in any stage of development have 

 more than three osseous pieces been observed in the basis of 

 the cranium from the so-called " vomer" to the " basi-occipital" 

 included. The assumed " connation " of the centrums of the 

 pre- and post-sphenoids, as held by Professor Owen, has at 

 present no support from embryology ; the missing centrum or 

 centrums must therefore be accounted for otherwise than by a 

 hypothetical division of the " basi-sphenoid." Professor Owen 

 appears, indeed, to a certain extent to admit this, for in 

 certain fishes he considers the symmetrical Y-shaped ossicle 

 marked in his diagrams 9 1 , and superimposed on the pre- 

 sphenoidal process of his basi-sphenoid, as the central part ; 

 while that process itself he holds to be the capsular portion 

 of the ossified notochord. 



That mutual elongation and overlapping of the cranial 

 centrums formerly alluded to is strongly marked in fishes, the 



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