SS6 ON THE THEORY OF THE VERTEBRATE SKULL 



precisely with the line of junction between the rostrum of the 

 basisphenoid 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 inner 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 

 of the squamosal is seen entering into its outer boundary above the 

 mastoid. On regarding the turtle's skull in the same way, there is 

 seen, occupying the same position, the bone which Cuvier, as I 

 venture to think, most unfortunately, named " mastoid." But if 

 the arguments brought forward above be, as I believe with Kall- 

 mann, they are, irrefragable, this bone cannot be the mastoid ; and 

 I can discover no valid reason why it should not be regarded as 

 what its position and relations naturally suggest it to be — the 

 squamosal. Its connections with the mastoid, petrosal, and quadratum 

 are essentially the same as those of the squamosal in the bird and the 

 mammal. The quadratum and articulare of the turtle are on all 

 hands admitted to be the homologues of the similarly-named bones, 

 in the bird, and therefore all the reasonings which applied to the 

 one apply to the other. When the petrosal, mastoid, and squamosal 

 are determined in the turtle, they are determined in all the Reptilia. 

 But the Crocodilia, Lacertilia, and Ophidia differ from the turtle and 

 Chelonia generally, in that their mastoid is, as in the bird, anchylosed 

 with the exoccipital. The squamosal, again, which in the Crocodilia 

 essentially resembles that of the turtle, becomes a slender and elon- 



