THE STRUCTURE OF THE PIKE'S SKULL. 169 



The last ossification of the craniofacial axis is a depressed 

 bone Vo, thicker in front than behind, which fits on to the under- 

 surface of the anterior half of the bone just described, and ex- 

 tends beyond it to the front end of the snout. The under-surface 

 of this bone is free, enters into the middle of the roof of the 

 palate, and bears teeth. 



In comparing the cranio-facial axis of the Pike with that of 

 Man, two pair of bones appear, at once, to correspond so closely 

 that no reasonable doubt can be entertained as to their homology. 

 These are the posterior and anterior bones of the series in each 

 case. The former, in its relation to the spinal column, to the 

 medulla oblongata, and to the lateral arches of the skull of the 

 Pike, is precisely comparable with the basi-occipital of Man ; 

 while the anterior bone as exactly answers to the vomer of man ; 

 except that the fish, being devoid of any communication between 

 the olfactory chambers and the cavity of the mo 7 dh, the vomer 

 has a different form, and has of course no relation to nasal 

 passages. 



Again, it seems obvious that the ethmoid is represented only 

 by cartilage, as in the foetal state of the human skull, for there 

 is no ossification in that portion of the cranio-facial axis which 

 lies between the olfactory sacs. 



And the like appears to be true of the presphenoid, for all 

 that vertical plate-like portion of the cranio-facial axis which 

 lies between the orbits, and beneath the peduncles of the 

 olfactory lobes, and in front of the crossing of the optic nerves, 

 is merely cartilaginous. 



The Y-shaped bone forms part of the front and side walls 

 of the pituitary fossa, and its upper prolongations are connected 

 behind with the bones Pr.O, and with the floor of the cranial 

 cavity. In this floor, the long cartilaginous plate, already men- 

 tioned, constitutes the hinder boundary of the fossa, and sepa- 

 rates the Y-shaped bone from the basi-occipital. Now, the 

 proper basi-sphenoid (that is to say, the central ossification 

 taken apart from the linguist) forms the front boundary of the 

 pituitary fossa in Man, but extends obliquely downwards in 

 front of it as the stem of the Y-shaped bone does in the Pike. 



