d2 cerebral development 



a progress from simple cartilaginous boxes to cartilaginous 

 boxes covered with osseous plates, that performed the func- 

 tions, whether active or passive, of internal bones ; and then 

 from external plates to the interior bones which the plates 

 had previously represented, and whose proper work they had 

 done. 



The principle which rendered it necessary that the divi- 

 sions which exist in the dermal skulls of the first ganoids 

 should so closely correspond with the divisions which exist 

 in the internal skulls of the osseous fishes of a greatly later 

 period, does not seem to lie far from the surface. Of the 

 solid parts of the ichthyic head, a certain set of pieces afibrd 

 protection to the brain and cerebral nerves, and to some of the 

 organs of the senses, such as those of seeing and hearing ; 

 while another certain set of pieces constitute the framework 

 through which an important class of functions, manducatory 

 and respiratory, are performed. The protective bones of 

 merely passive function are fixed, whereas the bones of active 

 function, such as the jaws, the osseous framework of the 

 opercules, and the hyoid bones, are to the necessary extent 

 free, i. e. capable of independent motion. Of course, the de- 

 tached character necessary to the free cerebral bones would 

 be equally necessary in cerebral plates united derm ally to the 

 pieces of the cartilaginous framework which performed in 

 the ancient fish the functions of these free bones ; and hence 

 jaw plates, opercular plates, and hyoid plates, whose homolo- 

 gical relation with recent jaws and opercular and hyoid bones 

 cannot be mistaken. They were operative in performing 

 identical mechanical functions, and had to exist, in conse- 

 quence, in identical mechanical conditions. And an equally 

 simple, though somewhat difierent principle, seems to have 

 regulated the divisions of the fixed cranial bucklers of the 

 Old Red ganoids, and to have determined their homologies 

 with the fixed cerebral bones of the osseous fishes. 



