

405 



divided into a superior and an inferior portion, which meet at an 

 angle, the superior portion articulating with the parietal (and 

 squamosal), the inferior with the alisphenoid. Inferiorly, the 

 petrous articulates with the basisphenoid, and, to a small extent, 

 with the basioccipital. Posteriorly it articulates with a bone 

 through which the pneumogastric passes, and which, guided by 

 the analogy of most Reptilia, of Amphibia, and of birds, I believe 

 to represent the coalesced or connate mastoid and exoccipital. The 

 bone lodges the anterior part of the auditory labyrinth ; its middle 

 region corresponds with the middle of the mesencephalon. But as 

 it does not separate the auditory organ from the cavity of the skull, 

 it naturally presents no foramina corresponding with those through 

 which the portio dura and portio mollis pass in Abranchiate Ferte- 

 brata and Amphibia. There is one relation of the petrosal in the 

 fish, however, in which it seems to differ from that of any of the ovi- 

 parous Fertebrata hitherto described. Superiorly and posteriorly, 

 in fact, it does not unite with the supraoccipital, which is small, 

 comparatively insignificant, and occupies the middle of the posterior 

 and superior region of the skull ; but with a large and distinct bone 

 which forms the internal of the two posterolateral angles of the 

 skull, unites internally with the supraoccipital, anteriorly with the 

 parietal and petrosal, inferiorly with the conjoined mastoid and ex- 

 occipital. It is the bone which was called "occipital externe" by 

 Cuvier ; and he and others have supposed it to be the homologue of 

 that bone in the turtle which, following Hallmann, I have endea- 

 voured to prove to be the mastoid. As I have already shown, the 

 true mastoid of the fish must be sought elsewhere, and consequently 

 the Cuvierian determination is inadmissible. And I must confess, 

 that if our comparisons be confined to adult Vertebrata, the only 

 conclusion which can be arrived at seems to be, that this bone is 

 peculiar to fishes. 



But a remarkable and interesting observation of Rathke, com- 

 bined with the peculiar structure of the skull of the chick described 

 above, leads me to believe that when their development is fully worked 

 out, we shall find a distinct representative of this bone in many, 

 if not all, vertebrate crania. 



In his account of the development of Coluber natrix (see Note 

 IV.), Rathke states that three centres of ossification make their 



