178 ON THE MORPHOLOGICAL CONSTITUTION OF 



sphenoidal centrum being dovetailed into and elongated beneath 

 the occipital behind, and above the ethmoidal (" vomer ") in 

 front. The manner in which the anterior elongated portion 

 of the post-sphenoidal centrum of the bird elevates and carries 

 on its upper surface the compressed pre-sphenoidal centrum 

 has already been stated ; and I must again observe that it 

 appears to me that the pre-sphenoidal centrum exists in certain 

 fishes only in the form of a bar of cartilage a portion of the 

 "primordial cranium" situated on the upper surface of the 

 anterior prolongation of the post-sphenoidal centrum, and 

 terminating on the upper surface of the ethmoidal centrum or 

 so-called " vomer," and that in fishes with an " ossified orbital 

 septum " or " cranial ethmoid," it is to be recognised in the 

 half- ossified cartilaginous mass which unites the right and left 

 plates of that " septum," and which have been already indi- 

 cated as its corresponding neurapophyses. The pre-sphenoidal 

 is an undeveloped centrum in the fish, retaining more or less 

 of its w primordial" texture and form, and elevated, therefore, 

 above, or carried inwards, so as to be covered by the fully- 

 developed ethmoidal and post-sphenoidal centrums. 



I am acquainted with no example of a fully-developed 

 temporal centrum. It is represented in the "primordial 

 cranium" by the quadrilateral cartilaginous plate, bounded 

 laterally by the ear-capsules, behind by the portion corre- 

 sponding to the cartilaginous lateral occipitals, and in front by 

 the part in which the post-sphenoidal centrum first appears. 

 In all vertebrate animals this portion of the basis of the 

 primordial cranium is of great comparative extent, and is 

 encroached upon by the advancing ossification of the occipital 

 and post-sphenoidal centrums in modes which vary in the 

 different vertebrate forms. In mammals, the occipital advances 

 into it at the expense of the post-sphenoidal centrum. In 

 birds and fishes the post-sphenoidal passes more backwards. 

 In the reptiles the two centrums appear to share it equally. 



