246 ON THE STRUCTURE OF THE SKULL. 



(Fig. 88, A), and exhibiting a corresponding extent of the cranium. 

 The three segments of the basi-cranial axis are at once recog- 

 nisable and identifiable with the basi-occipital, basi-sphenoid, and 

 presphenoid of the Bird ; but the basi-sphenoid is truncated at its 

 anterior end, as in the Crocodile, not produced into a long beak, 

 as in Birds and many Lizards. The ex-occipital and supra-occi- 

 pital bones, again, have all the relations of those of the Ostrich, 

 and are universally admitted to be the homolognes of the latter. 



In the Ostrich, as we have seen (Fig. S8, A), there lies in front 

 of the ex-occipitals a large bony mass, composed of the confluent 

 opisthotic, epiotic, and pro-otic bones. The inner face of the 

 single periotic bone thus formed is divided into two surfaces, one 

 anterior and one posterior, by a ridge which runs somewhat 

 obliquely from above downwards and forwards. The anterior 

 surface is concave, looks somewhat forwards, articulates in front 

 with the alisphenoid, and contains no part of the organ of hear- 

 ing ; the third division of the trigeminal nerve passes out in 

 front of it. The posterior surface presents, inferiorly, the aper- 

 tures for the jportio dura and the jportio mollis ; superiorly and in 

 front, a fossa arched over by the anterior vertical semicircular 

 canal ; while, superiorly and behind, it contains the posterior 

 vertical semicircular canal. Between the posterior edge of this 

 division of the bone and the ex-occipital the eighth pair of 

 nerves leaves the skull. 



In the Beaver (Fig. 97), there is a single mass of bone not 

 dissimilar in form and proportional size, which has always been 

 admitted to be the homologue of the pars jpetrosa and jmrs mas- 

 toidea of Man, and the general relations and characters of which 

 may be described in exactly the same terms. The inner face is 

 divided into two surfaces, one anterior and one posterior, by a 

 ridge which runs, somewhat obliquely, from above downwards and 

 forwards. The anterior surface is concave, and looks slightly 

 forwards ; it articulates in front with a bone which, as all agree, 

 corresponds with the alisphenoid of Man, and lies behind the 

 exit of the third division of the trigeminal. The posterior divi- 

 sion presents, inferiorly, the apertures for the portio dura and 

 portio mollis ; superiorly and in front, a fossa arched over by the 

 anterior vertical semicircular canal ; while superiorly and behind 



