STRUCTURE OF FISHES. 419 



fishes remains in the incipient stage of ossifi- 

 cation, being more or less cartilaginous ; and 

 where the ossific process has begun, it has not 

 advanced the length of producing union between 

 the pieces formed from the separate centres of 

 ossification. Where they meet without uniting, 

 they form no sutures, but overlap one another. 

 Thus the bony structures are detached, and often 

 completely isolated ; affording to the physiolo- 

 gist an opportunity of studying the earlier stages 

 of this interesting process, and marking with 

 distinctness the number of the elements of each 

 bone, and the relative situations of their centres. 

 This knowledge is more especially of importance 

 towards understanding the formation and con- 

 nections of the bones of the head, which are 

 very numerous and complicated ; and the inves- 

 tigation of which has been prosecuted with ex- 

 traordinary diligence by Geoffroy St. Hilaire 

 and other continental zootomists. 



It is here, more especially, that we obtain the 

 clearest evidence of the derivation of the cranial 

 bones from vertebrae analogous to those of the 

 spine. The occipital bone, in particular, corre- 

 sponds to a spinal vertebra in all its essential 

 elements. In many fishes, the body of this 

 bone, being lengthened out to form the posterior 

 part of the basis of the skull, becomes the basi- 

 lar portion. We find, on its posterior surface, 



