Mr. T. II. lln\ley on the Theory of the Vertebrate Skull. 125 



the mammal (as all anatomists do), can deny that the bone in ques- 

 tion is also the petrosal, and affirm it to be an alisphenoid. The 

 general adoption of such a view would, I do not hesitate to say, 

 throw the Theory of the Skull into a state of hopeless confusion, 

 and render a consistent terminology impossible. Where then is the 

 alisphenoid ? I reply, that it is unossified. The posterior portion 

 of the cartilaginous side-wall of the skull, in fact, unites with the 

 parietal, the petrosal, and the basisphenoid, just in the same way as 

 the bony alisphenoid of the bird unites with those bones. Further- 

 more, as in the bird, it bounds the foramen for the third division of 

 the trigeminal nerve anteriorly, and is specially perforated by the 

 second division of the fifth, while the optic and the other divisions 

 of the fifth pass out in front of or through its anterior margin. 



Not only is the alisphenoid cartilaginous, but the orbitosphenoid 

 is in the same condition, and a great vertical plate of cartilage re- 

 presents the whole anterior part of the craniofacial axis, or the pre- 

 sphenoid and ethmovomerine bones*. It has been imagined, indeed, 

 that the rostrum-like termination of the basisphenoid represents 

 the presphenoid, but I think this comes of studying dry skulls. 

 Those who compare a section of the fresh skull of a turtle with the 

 like section of the skull of a lamb, will hardly fail to admit that the 

 rostrum of the basisphenoid in the turtle is exactly represented by 

 that part of the sheep's basisphenoid which forms the anterior and 

 inferior boundary of the sella turcica, and that the suture between 

 the basisphenoid and the presphenoid in the sheep corresponds pre- 

 cisely with the line of junction between the rostrum of the basi- 

 sphenoid and the presphenoidal cartilage in the turtle. 



Connected with the posterior edge of the petrosal by the carti- 

 laginous plate, which has been referred to above, and between this 

 and the exoccipital, there appears, on the iimer aspect of the longi- 

 tudinal section of the turtle's skull, a narrow plate of bone connected 

 above with the supraoccipital, behind with the exoccipital, below 

 with the basioccipital, and leaving between its posterior margin and 

 the exoccipital an aperture whereby the par vagum leaves the skull. 

 In fact, except in being separated from the petrosal by cartilage, this 

 bone presents all the characters of the mastoid of the bird, which it 

 further resembles in forming one-half of the circumference of the 

 fenestra ovalis. In other respects it is more like the mastoid of the 

 sheep, for it is not anchylosed with the exoccipital ; it is produced 

 externally into a great bony apophysis, which gives attachment to 

 the representative of the digastric muscle ; and it is largely visible 

 external to the exoccipital, when the skull is viewed from behind. 

 Indeed, the resemblance to the mastoid of the mammal is more 

 striking than that to the corresponding bone in the bird. And I 

 think it is hardly possible for any unprejudiced person to rise from 

 the comparison of the chelonian skull with that of the mammal, with 

 any doubt on his mind as to the homology of the two bones. 



When the sheep's skull is viewed from behind, the posterior half 



* Compare Kolliker's account of the primordial skull of a young turtle in the 

 ' Bericht von der Kbnigl. Zool Anstalt zu WUrzburg,' 1849. 



