THE TELEOSTEI. 153 



read a.s.c, anterior, and p.s.c, posterior semicircular canal; .r, the 

 parasphenoid ; y, the basisphenoid ; J'o, the vomer; P., the pituitary 

 fossa. 



sphenoidal and orbitosplienoidal regions commonly, but 

 not always, remain unossified. 



In most osseotts fishes, the base of the skull in front of 

 the basisphenoid is greatly compressed from side to side, 

 and forms an interorbital septum (I. Or.). The anterior 

 moiety of the cranial cavity is conseqiiently reduced to a 

 comparatively narrow passage above the septum (Fig. 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 diminishes forwards. The ethmoidal 

 cartilage usually remains unossified, but sometimes, as in 

 the Pike, ossification may take place in it. (Fig. 44, 3, 3.) The 

 antorbital, or lateral ethmoidal, processes of the primordial 

 cranium ossify, and give rise to the prefrontal bones (Frf.). 

 The postorbital processes also ossify as postfrontals 

 [Ptf.). The iipper and posterior part of the primordial 

 cranium exhibits five processes — one postero-median, two 

 postero-lateral, and two postero-external. The postero- 

 median ossifies as part of the supra-occipital [S.O.) The 

 postero-lateral ossifies as part of the epiotic (Ep.O.), which 

 lies upon the summit of the superior vertical semicircular 

 canal. The postero-external closely con-esponds with the 

 squamosal of the higher Vertebrata in position ; bttt, as a 

 cartilage bone, it corresponds with an ossification of the 

 capsule of the ear, called pterotic in the higher Vertebrata. 

 Not unf requently, as in the Cod, for example, the opisthotic 

 (Op.O.) is a distinct bone, and enters into the formation of 

 the postero-external process. The pro-otic (Pr.O.) is always 

 a well-developed bone, and occupies its regular place, in 

 front of the anterior vertical semicircular canal, and behind 

 the exit of the trigeminal nerve. 



In addition to these cartilage bones, the brain- case of 

 osseous fishes is additionally defended by numerous mem- 

 brane bones. These are, on the roof of the skull — 



1. The parietal bones [Pa.), which sometimes meet in a 



