132 



THE ANATOMY OF VERTEBRATED ANIMALS. 



tebrse, and bear the fin-rays of the anal, and, in part, of the 

 caudal fin. 



The 2'eleostei differ very much in the extent to which the 

 primordial cranium persists throughout life. Sometimes, as 

 in the Pike (Figs. 44 and 45), it grows with the growth of ilie 



e^msmsa. 



I.O'r. AS. JiS Flirt. 



FiQ. 44. — The cartilagiDous cranium of the Hke (Esox lucius), vrith its intriosic fssifica- 

 tions; viewed, A, from above ; B, frombeloTv; C, from the left side : i\^, -^, nasatfosaee; 

 /. Or, Interorbital septum ; a, groove for the median ridge of the parasphenoid ; &, canal 

 for the orbital muscles. Sq., wrongly so marked, is the Pterotic. Y. and VIII. mark 

 the exits of the fifth and pneumogastric nerves ; 8, 8, small ossifications of the rostrum. 



fish, and only becomes partially ossified ; in other cases it al- 

 most disappears. A basi-occipital {J5. 0.), ex-ocoipital (E. 0.), 

 and supra-occipital {S. 0.) bone are developed in it, and form a 

 complete occipital segment. The proper basi-sphenoid {£S) 

 bone is always a very small, and usually somewhat Y-shaped, 

 bone. The alisphenoids (AS.) sometimes are and sometimes 

 are not developed. The presphenoidal and orbitosphenoidal 

 regions commonly, but not always, remain unossified. 



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

 basisphenoid is greatly compressed from side to side, and 



