THE TELEOSTEI. 



133 



forms an interorbital septum [Z. Or.). 

 the cranial cavity is consequently re- 

 duced to a comparatively narrow pas- 

 sage above the septum (t'ig. 45). In 

 the Siluroid and Cyprinoid fishes, 

 however, this septum is not formed, 

 and the cranial cavity is of nearly equal 

 size throughout, or gradually diminish- 

 ing forward. The ethmoidal cartilage 

 usually remains unossified, but Rome- 

 times, as in the Pike, ossifica,tion may 

 take place in it. (Fig. 44, 3, 3.) The 

 antorbital, or lateral ethmoidal, pro- 

 cesses of the primordial cranium os- 

 sify, and give rise to the prefrontal 

 bones [JPrf.). The postorbital pro- 

 cesses also ossify as postfrontals [Ptf.). 

 The upper and posterior part of the 

 primordial cranium exhibits five pro- 

 cesses — one postero-median, two pos- 

 tero-lateral, and two postero-external. 

 The postero-median ossifies as part of 

 the supra-occipital (>S. 0.). The pos- 

 tero-lateral ossifies as part of the epi- 

 otic {Ej). 0.), which lies upon the sum- 

 mit of the superior vertical semicircular 

 canal. The postero-external closely 

 corresponds with the squamosal of the 

 higher Vertebrata in position ; but, as 

 a cartilage bone, it corresponds with 

 an ossification of the capsule of the ear, 

 called pterotlc in the higher Vertebrata, 

 Not unfrequently, as in the Cod, for 

 example, the opisthotic ( Op. 0.) is a 

 distinct bone, and enters into the for- 

 mation of the postero-external process. 

 The prootic (^JPr.O.) is always a well- 

 developed bone, and occupies its regu- 

 lar place, in front of the anterior ver- 

 tical semicircular canal, and behind the 

 exit of the trigeminal nerve. 



In addition to these cartilaore 

 bones, the brain-case of osseous fishes 

 is additionally defended by numer- 



The anterior moiety of 



Fig. 45. — Longitudinal and ver- 

 tical section of a fi-esh Pike's 

 skull. — The cut surface of car- 

 tilage is dotted. For S. V. C. 

 and P.V.C.^ read a.s.c.^ an- 

 terior, and 'p.s.G.y posterior 

 semicii'cular canal ; a, tha 

 parasphenoid ; y, the hasi- 

 sphenoid; Fo, the vomer 

 P., the pituitary fossa. 



