Mr. T. 11. ITuxlcy on the Theory of the Vertebrate Skull. 427 



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

 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 Kallmann, 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 VertebratUy 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 natrixy 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 represents the mastoid, and it is 

 most interesting to find it, in this early condition, as distinct as in 

 the Chelonian. 



The superior ossification has only to increase in size and remain 

 distinct in the same way as the mastoid of the turtle remains di- 

 stinct, to occupy the precise position of the "occipital externe" of 

 the fish. But, further, it is most important to remark, that when 

 this primarily distinct bone has coalesced with the supraoccipital, 

 it stands in just the same relation to that bone, to the petrosal, to 

 the mastoid and to the semicircular canals, in the snake, as that 

 lateral element, early confluent or connate with the supraoccipital 

 in the chick, which I have termed the "os epioticum." I believe, 

 then, that this "os epioticum," distinct in the young snake, but 

 afterwards confluent with the supraoccipital, and becoming what may 

 be termed the epiotic ala of that bone in the adult, is the homologue 

 of the corresponding bone, or confluent ala of the supraoccipital, in 

 birds and reptiles, while in the fish it remains distinct, and constitutes 

 the " occipital externe." 



On examining the region in which these bones (those of the 

 palato-suspensorial apparatus) are eventually found, in an embryonic 

 fish, I discovered, in their place, a delicate inverted cartilaginous 

 arch, attached anteriorly, by a very slender pedicle, to the angles of 



