46 PROCEEDINGS OF THE ACADEMY OF [1894. 



fifth uerve is irreconcilable, as in the case of the fish, with the idea 

 that it is the homologue of the upper prootic portion of the human 

 pars petrosa. On the other hand, if the bone No. 6 in the reptile be 

 regarded as the homologue of the alisphenoid in Man the difiiculty 

 presents itself that the former enters into the formation of the fenestra 

 ovalis and transmits in the reptile the filaments of the facial and 

 acoustic nerves, which the alisphenoid of INIan never does, the fenestra 

 rotunda and the nerves being confined to the pars petrosa. With 

 the view of reconciling these difiiculties the bone No. 6 in the reptile 

 has been regarded by some anatomists as consisting really of two 

 bones fused into one, the anterior and posterior parts being viewed 

 respectively as alisphenoid and prootic bones. 



The study of the development of the reptilian skull offers some 

 confirmation of this view, since as a matter of fact, according to 

 Parker,"'' the bone No. 6 in the snake develops from two centres, the 

 anterior of which is regarded as the alisphenoid, the posterior as the 

 prootic. If the latter view be accepted the result of development in 

 the skull of the suake is very different from that in Man, since the 

 prootic ossification, instead of combining with the opisthotic to form 

 the pars petrosa, unites with the alisphenoid to form one bone. 

 Further, it does not follow because the bone No. 6 develops from two 

 centers of ossification that it must necessarily be regarded as consist- 

 ing of two distinct bones, any more than the basi sphenoid must be 

 regarded as consisting of three bones because it develops from three 

 centres.^" It seems to the author that the view most reconcilable 

 with the facts of development as well as those relating to the adult 

 condition of the skull is to regard the bone No. 6 in the reptile as the 

 homologue of the bone so numbered in the fish and of the alisphenoid 

 in Man. 



In the absence of a pars petrosa in the skull of the reptile and 

 bird some other bone or bones must fulfil the functions of that bone 

 in supporting and protecting the labyrinth and in transmitting the 

 facial and acoustic nerves. These functions are filled in the reptile 

 more or less by the bones Nos. 6, 4, 2, 3, which we regard as the 

 alisphenoid, external occipital, ex-occipital and supra-occipital, or. 



29 The Morphology of the Skull, London, 1877, p. 204. 



^ One for the mediau basisphenoid, two for the symmetrical basitemporals, 

 the homologiies of the liugulae sphenoidal es of Man. 



