'S6o ON THE THEORY OF THE VERTEBRATE SKULL 



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 pneumo- 

 gastric 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 

 ho foramina corresponding with those through which the portio dura 

 and portio mollis pass in Abranchiate Vertebrata 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 oviparous Vertebrata 

 hitherto described. Superiorly and posteriorly, in fact, it does not 

 unite with the supraoccipital, which is small, comparatively in- 

 significant, 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 exoccipital. 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 Kallmann, I have endeavoured 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 

 appearance in that part of the cartilaginous wall of the cranium 

 which immediately surrounds the auditory labyrinth. One of these 

 is anterior, and becomes the petrosal ; one is posterior, and eventually 

 unites with the exoccipital ; the third is superior, and in the end 

 ~ coalesces with the supraoccipital. The posterior ossification clearly 



