THE THEORY OF THE VERTEBRATE SKULL. 30 L 



15. There is not a shadow of evidence that the mandibular 

 and hyoidean arches suffer any shifting of position from before 

 backwards, in the course of their development; but the extre- 

 mities of those arches which are attached to the skull undergo 

 very singular metamorphoses, the effect of which is, that the 

 dentary part of the mandible is brought into closer connection 

 with the skull the higher we ascend in the Vertebrate series. 

 Thus, in the Fish it is separated from the skull by the hyoinan- 

 dibular, quadrate, and articular bones ; in the Reptile by the 

 quadrate and articular ; while in the Mammal the quadrate 

 and the articular are metamorphosed into the incus and the 

 malleus, and the dentary comes close to the skull, articulating 

 with the squamosal. 



These are, I believe, the most important facts regarding the 

 structure and development of the skull, which may now be 

 regarded as well established. If we inquire how they bear 

 upon theories of the skull, it will be obvious that they place the 

 doctrine of the unity of organization of the vertebrate skull 

 upon a perfectly sure and stable footing, while they appear to 

 me, as clearly, to negative the hypothesis that the skull is, in 

 any sense, a modification of vertebrae. 



But though the skull has not a vertebral structure, and in 

 its membranous and cartilaginous states is not even segmented, 

 it assumes a very definite segmentation in its completely ossified 

 state. 



In every well-ossified cranium there is, assuredly, an occipital 

 segment (" Ear Vertebra " of Oken), formed by the basi-occipital, 

 ex-occipitals, and supra-occipital ; a parietal segment (" Jaw 

 Vertebra " of Oken), constituted by the basi-sphenoid, alisphe- 

 noid, and parietals ; a frontal segment (" Eye Vertebra " of 

 Oken), composed of the presphenoid, orbito-sphenoids, and 

 frontals ; and a nasal segment (" Nasal Vertebra " of Oken), 

 formed by the ethmoid, prefrontals, turbinals, nasals, and 

 vomer. 



Leave out the hypothetical considerations that these seg- 

 ments are equivalent to one another, and that they are homo- 

 logous with vertebra?, and Oken's expression of the broad facts 



