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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 Hall- 

 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 squa- 

 mosal. Its connexions 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 arid elon- 

 gated bone in the Lacertilia, and still more in the Ophidia, in 

 which the quadratum is carried at its extremity *. 



In the Amphibia the petrous and mastoid have the same relations 

 as in the Reptilia ; but it is interesting to remark, that in some Am- 

 phibia the anterior margins of the petrosal encroach upon the lateral 



* See for the manner in which this is brought about, Rathke's ' Entwick. d. 

 Natter.' Rathke, it should be said, regards this bone as the tympanicum, but its 

 primitive place and mode of origin are those of the squamosal of the mammal. 



