134 



THE ANATOMY OF VERTEBRA TED ANIMALS. 



ous membrane bones. These are, on the roof of the 

 skull— 



1. The parietal bones (I'a.), which sometimes meet in a 

 sagittal suture, as in most of the higher Vertebrata, but are 

 very generally separated by the junction of the Jrontals with 

 the supra-occipital. 



2. The large frontals (-Z'r.), which may or may not unite 

 into one. 



3. The nasal bones (JVa.), apparently replaced in the PJke 

 by the bones 1 and 2. 



Fio. 46. — Side and upper viewa of the skull cf a Pike {Esox luciuB\ without the facial or 

 eupra-orbital bones : ?/, the basisphenoid; is, the alispheuoid; a, the articular fecet for 

 the hyomandibular bone. 



The under-surface of the skull possesses two membrane 

 bones : in front the vomer ( Vb.), and, behind, the huge para- 

 sphenoid (x, x), which ensheathes all the basis cranii, from 

 the basi-occipital to the vomer. 



A supra-orbital bone (& Or.) is the only membrane bone 

 attached to the sides of the brain-case. Two premaxillary 

 bones [Pmx.) are attached, sometimes closely, sometimes 

 loosely, to the anterior extremity of the cranium ; and behind 

 these are the maxillse (Jfo.), which are sometimes large and 

 single, as in the Cyprinoid fishes, but may become subdivided, 

 or be reduced to mere styliform supports for cirri, as in many 

 Siluroid fishes. In most osseous fishes the maxillae take little 

 or no share in the formation of the gape, which is boimded 

 above by the backwardly-extended premaxillfe. 



The palato-quadrate and hyomandibular have essentially 

 the same structure and arrangement as in Lepidosteus and 



