ON THE THEORY OF THE VERTEBRATE SKULL 



559^ 



from the common plan, and therefore forms a better type for 

 comparison with the skulls of other Vertebrata than that of any 

 acanthopterygian or ganoid fish. 



The craniofacial axis presents only four distinguishable bones.. 

 Behind, is the short basioccipital, with its cup for articulation with 

 the first vertebra of the spinal column. In front of this is a greatly 

 elongated bone, which, as in the bird, sends a process as far as the 

 vomer, and forms the greater part of the axis of the skull ; and which, 

 I believe, represents, as in the bird, the basisphenoid and more or 

 less of the presphenoid. The short vomer terminates the craniofacial 

 axis anteriorly, and bears upon its upper surface a vertical septum,, 

 which, as in the bird, expands into a broad plate above, and is the 

 ethmoid. 



The orbitosphenoids, united below, spring from the upper and 

 anterior part of the presphenoid. Behind them the lateral walls of 



Fig. 6. — Longitudinal section of the Skull of a Carp (Cyprinus carpio). 



the skull are formed by the alisphenoid. These bones have the 

 same essential relations as in the bird, for the olfactory nerves pass 

 out of the skull over, and in front of, the orbito-sphenoids ; the optic 

 nerves make their exit behind and beneath these and the alisphenoid,, 

 while the trigeminal makes its exit behind the posterior edge of the 

 alisphenoid. When viewed from within, the foramen ovale is seen to 

 be as in the bird, a mere conjugational foramen between the alisphe- 

 noid and the bone which follows it ; and on an external view, the 

 third division of the trigeminal is seen to pass entirely in front of the 

 last-named bone. 



The minutest scrutiny of the relations of this bone only strengthens 

 the conviction suggested by the first view of it, that it is the homo- 

 logue of the petrosal of birds, and therefore of mammals and reptiles. 

 As in the bird, the anterior margin of the fish's petrosal is divided 

 into a superior and an inferior portion, which meets at an angle,. 



