THE STRUCTURE OF THE PIKE'S SKULL. 173 



The third of the periotic ossifications — the opisthotic — seems 

 at first not to be discoverable in the Pike. But in some speci- 

 mens of this fish, and in a great many other fishes, there is a 

 distinct bone (which is particularly large and conspicuous in the 

 Gadidce) connected below and behind with the ex-occipital, in 

 front with the pro-otic, internally and behind with the epiotic, 

 and externally with the squamosal. It enters especially into the 

 outer and posterior wall of the labyrinth, and protects a great 

 part of the external semicircular canal, sharing this function 

 with the pro-otic. 



As there is neither fenestra ovalis nor rotunda in the fish, 

 and as it is by no means certain whether the sacculus does or 

 does not correspond with the cochlea of the higher Vertebrata, 

 some of the best marks by which the opisthotic may be identi- 

 fied are wanting ; but the relations of this bone to the other 

 periotic ossifications seem to me to be decisive as to its real 

 nature. 



The periotic bones being thus identified, they are all elimi- 

 nated from comparison with the proper supero-lateral consti- 

 tuents of the cranial arches. And there remains only one bone 

 in the lateral walls of the Pike's cranial cavity which can answer 

 to any of these, which is that marked z in Figure QS, A.S. in the 

 other figures. 



This bone has the form of a triangle, with its apex turned 

 downwards. The hinder side of the triangle abuts against the 

 anterior margin of the pro-otic, and closes the trigeminal notch 

 in that bone anteriorly. The front margin ends in the cartila- 

 ginous side walls of the skull ; the apex approaches, but stops a 

 little short of, the lateral wing of the Y-shaped bone, or basi- 

 sphenoid. The relations of this bone are therefore essentially 

 those of the alisphenoid, though I think it quite possible that 

 the orbito-sphenoicl may, to a certain extent, be represented by 

 its anterior portion. 



The bones which enter into the roof of the skull (Fig. 69) 

 remain for consideration. Of these, the supra-occipital, which 

 has no additional constituent comparable to 8.0' in Man, has 

 already been mentioned. It articulates in front with two very 

 large and long bones (Fr), separated by a median suture, which 



