THE STRUCTURE OF THE PIKE'S SKULL. 175 



homologues. These are, firstly, the bones called post-frontal, 

 Pt.f., which form the posterior, superior, and external angles of 

 the orbits, and are wedged in between the alisphenoids and the 

 pro-otics ; secondly, the bones marked 1 and 2, developed upon 

 the ethmoidal cartilage external to the points of the frontals. 

 The pair 1,1, which immediately overhang the external nares, 

 are probably to be regarded as the nasals of Man ; but the nature 

 of the second pair, 2,2, which lie internal to them, and extend to 

 the end of the snout, is doubtful. Still less does there appear 

 any reason to identify the bones 3,3, which are minute tri- 

 angular ossifications in the substance of the cartilage between 

 the bones 2,2 and the vomer, with any which exist in man. I 

 consider them to be peculiar to the fish. 



And now to sum up, in a few words, the structure of the 

 brain-case of the Fish. We find, as in Man, a posterior, occi- 

 pital, segment, consisting of basi-occipital, ex-occipital, and supra- 

 occipital ; a middle, parietal, segment, consisting, as in Man, of 

 a basi-sphenoid, alisphenoids, and parietals, but in which the 

 latter, in consequence of the disproportionate size of the frontals, 

 are thrown far back out of connection with the alisphenoids ; 

 and, finally, an anterior, or, frontal, segment, of which only the 

 frontals are separately distinguishable in the osseous state. 

 The orbito-sphenoids and the presphenoid are alike represented 

 only by cartilage and membrane, unless, indeed, as has been 

 suggested, a part of the alisphenoid may take the place of the 

 former bones. 



Of the bony elements connected with the sense-capsules in 

 Man, the pro-otic, opisthotic, and epiotic, together with the 

 squamosal, have been clearly identified in the Pike ; as have the 

 pre-frontals and the vomer. 



But certain bones present in Man have not been recognised 

 in the Fish ; while, on the other hand, certain bones present in 

 the Fish appear to have no representatives in Man. 



Thus, while the study of the cranial structure of the Man 

 and the Pike reveals a fundamental identity of composition 

 between the two, it demonstrates the existence of a no less 

 marked diversity, each type exhibiting structures and combina- 

 tions peculiar to itself. 



