SKELETON OF TELEOSTEI. 85 



constancy throughout this sub-class ; they often coalesce with, 

 and are no more separable from, the neighbouring or under- 

 lying cartilage-bones. All these bones have been topogra- 

 phically enumerated in Chapter IV. 



Many attempts have been made to classify the bones of 

 the Teleosteous skull, according to then' supposed relation to 

 eac?i other, or with the view to demonstrate the unity of 

 plan on which the skull has been built ; but in all either 

 the one or the other of the following two principles has been 

 followed : — 



A. The " vertebral doctrine " starts from the undeniable 

 fact that the skull is originally composed of several segments, 

 each of which is merely the modification of a vertebra. The 

 component parts of such a cranial segment are considered to 

 be homologous to those of a vertebra. Three, four, or five 

 cranial vertebrsie have been distinguished, all the various 

 bones of the fully-developed and ossified skull being referred, 

 without distinction as to their origin, to one or the other 

 of those vertebral segments. The idea of the typical unity 

 of the osseous framework of Vertebrates has been worked out 

 with the greatest originality and knowledge of detail, by 

 Oioen, who demonstrates that the fish-skull is composed of 

 four vertebrse. 



The bones of the fish-skull are, according to him, prunarily 

 divisible into those of the neuroskeleton, splanchnoskeldon, and 

 de7'mosJceleton. 



The bones of the ncuro- or proper endo-skeleton are 

 arranged in a series of four horizontally succeeding segments : 

 the occipital, parietal, frontal, and nasal vertebrae ; each seg- 

 ment consisting of an upper (neural) and a lower (haemal) 

 arch, with a common centre, and with diverging appendages. 



The neural arches of the four vertebrae, in their succession 

 from the occiput towards the snout, are : — 



1. JSpcncephalic arch, composed of the occipitals. 



