156 ON THE MORPHOLOGICAL CONSTITUTION OF 



extremity of the post-sphenoidal centrum, and in the plane of 

 the ethmoidal centrum in front. I regard, therefore, the 

 " os en ceinture " in the Batrachian, along with the anterior 

 segments of its " parieto-frontals," as consisting of the centrum, 

 neurapophyses, and meta-neurapophyses of the pre-sphenoidal 

 sclerotome ; and, therefore, also as homologous with that seg- 

 ment in the ophidian which includes its " frontals," but 

 exclusive of the elongated anterior prolongation of the post- 

 sphenoidal centrum. 



The Pre-sphenoidal Centrum and Neural Arch in the Fish. 

 The bone which predominates over every other in the cranium 

 of the fish is the so-called "principal frontal ;" which, how- 

 ever, as already stated, I do not regard as the frontal or 

 ethmoido-frontal of the mammal, but as a sphenoido-frontal. 

 It is the pre-sphenoidal meta-neurapophysis of the fish, pre- 

 senting all the relations of the corresponding bone or bones in 

 the bird, chelonian, and lizard, except that the ethmoido- 

 frontals anterior to it have coalesced in the middle line ; while 

 the ethmoidal neurapophyses have become so much developed, 

 exposed, and connected to it laterally, as to assume the position 

 of the so-called " nasals " and " pre-frontals " in the bird and 

 reptile. The enormous development of this bone in the fish 

 and bird appears to depend on the great bulk of the organs of 

 vision. There is, therefore, in both, an extended interorbital 

 space to be filled up. In the fish, as in the bird, this is vari- 

 ously effected by means of fibro-cartilage and bone. The 

 extreme forms of the interorbital arrangement may be illus- 

 trated by the gadoid and cyprinoid fishes. In the cod the 

 greater part of the so-called interorbital septum consists, as 

 in the chelonian and lizard, of a double fibrous membrane, 

 which extends upwards from the anterior prolongation of the 

 post-sphenoidal centrum to the margins of the mesial grooves 

 on the under surface of the sphenoido-frontal. The two 

 laminse of this membrane thus bound the sides of the 



