THE SKRLETOX OF THE VERTEBRATE HEAD. 141 



against tliis detenniiiation, but, on the contrary, are in accord- 

 ance with the union of the auditory capsules witli their cor- 

 responding neurapophyses, and the exit of the auditory nerve 

 from the enceplialic ca\dty in divisions. It must also be ob- 

 served, that if we arc to look, with Professor Owen, upon the 

 central lamina or bar of the mannnalian ethmoid as the result 

 of the mesial union of a pair of " pre-frontals," we must assign 

 a morphological reason for the co-existence of a mesial car- 

 tilaginous septum with divided "pre-frontals" of the reptile 

 and fish. 



I am also obliged to dissent from Professor Owen's deter- 

 mination of the so-called " ethmoid" of the bird as the mesially- 

 unitcd neurapophyses of its nasal vertebra. Apparently in- 

 fluenced by its usual designation, and restricted to liis own view 

 of its homology by his determination of the "basi-sphenoid" 

 as consisting of the connate centrums of the "mesencephalic" 

 and "prosencephalic" vertebra?, ]\Ir. Owen has in the bird, as 

 in the mammal, arranged this portion of his morphological 

 system in opposition to embryological facts. The two olfactory 

 nerves of the bird pass forward on each side of the so-called 

 " ethmoid" in shallow grooves ; in certain instances only do 

 they pass through notches or complete orifices formed by 

 osseous development from the two surfaces of the bone. The 

 two nerves in no instance pass forwards between the plates of 

 the bone in any part of their extent. At no period during 

 development are the olfactory nerves of the bird situated 

 mesiad of any part of this bone ; for it is originally a mesial 

 cartilaginous plate, a portion of the axis of the primordial 

 cranium, extending forwards and upwards from that part of 

 the primordial axis which, when ossified, constitutes the 

 anterior or acuminated extremity of the centrum of the post- 

 sphenoidal sclerotome. In the sequel I shall have to point 

 out that this bone in the bird, which anatomists have hitherto 

 looked upon as the " ethmoid," is, in fact, the body or centnim 



