THE SKELETON OF THE VERTEBRATE HEAP. IIP. 



later inquirers to be influenced by that a priori, or " transcen- 

 dental " method, characteristic of those German and Frencli 

 anatomists with wliom the subject originated. For my own 

 part, so far from coinciding in the received opinion, that the 

 number of seefments in the vertebrate head is the same in all 

 its forms, T believe that it varies. I shall state in the sequel 

 the grounds on which T hold the number of sclerotomes to var\' 

 slightly in the heads of the ordinary forms of vertebrata, 1 

 am, however, inclined to believe that there are indications 

 afforded by embryology and comparative anatomy, of the exist- 

 ence in certain forms of vertebrate head of a considerably greater 

 number of sclerotomes than has been generally supposed. 1 

 base this conjecture, first, on the system of cartilaginous nasal 

 segments in the cyclostomes ; and, secondly, on the circum- 

 stance that if the head is to be distinguished embrj^ologically 

 from the trunk, by the presence of " visceral laminaj " separ- 

 ated by clefts, then not only the cyclostomes, but the still 

 more remarkable branchiostoma indicate a number of cephalic 

 segments, and a form of vertebrate structure, of which, in the 

 present state of the science, it can only be said that such a 

 form is deducible from the vertebrate t^'pe. 



I recognise in the head of the iish, exclusive of the cyclo- 

 stomes, six sclerotomes ; in that of the amphibian and reptile 

 also six ; with the exception of the crocodiles, in which the 

 seventh is feebly developed ; in that of birds, six ; and in 

 that of mammals, exclusive of the proboscidians, seven. 



I find it more convenient to examine these sclerotomes 

 from before backwards ; and I distinguish them provisionally 

 liy the following designations : — 



1. RhINAL. 4. PRE-SPHENOroAL. 



2. VO.MERINE. 5. PoST-SrHENOIOAL. 



3. Ethmoidal. «".. Temporal. 



7. OcriPITAL. 



1 



