112 ANATOMY OF VERTEBRATES. 



which rests directly upon the presphenoid, and intercepts a 

 canal along which the olfactory ' crura ' are continued forward 

 to the prefrontals : the lateral parietes of this canal thus form 

 not only a complete, but a double bony partition between the 

 orbits. In the Shad a corresponding descending plate takes 

 the place of the orbitosphenoid. In most Acanthopteri an olfac- 

 tory groove is formed by short vertical descending plates from 

 the under surface of the frontal. The midfrontal is single in 

 the Plenronectidce, but has undergone more modification than 

 any of the preceding bones in connection with the general distor- 

 tion and loss of symmetry of the head : in the Halibut the right 

 posterior angle is truncated, and the rest of that side scooped out, 

 as it were, to form the large orbit of the right side : the left side 

 of the bone retains its normal form : a median crest, a continuation 

 of that upon the superoccipital, divides the two sides. 



The postfro?itals, figs. 75, 76, 83, 12, 12, obviously belong to the 

 same category of vertebral pieces as the mastoids, whose promi- 

 nent crest they partly underlie and complete, lending their aid in 

 the formation of the single (e. g. Cod, Salmon), or double, (e. g. 

 Pike) articular cavities for the tympanic pedicle : like the mastoids 

 they are ossified in and from the primitive cranial cartilage ; and 

 their inner surface is expanded, but this less frequently enters 

 into the formation of the cranial cavity : they form the posterior 

 boundary of the orbit ; are articulated below to the orbitosphenoid 

 and alisphenoid, above to the frontal, and by their posterior and 

 upper surfaces to the mastoid. 



The vomer, figs. 83, 84, 13, is wedged into the under part of 

 the presphenoid; its antero-lateral angles are articulated to the 

 prefrontals; its upper surface supports the nasal bone, sometimes 

 immediately, sometimes by an intervening ethmoidal cartilage. 

 The palatine bones abut against the expanded anterior part of 

 the vomer, the under side of which commonly supports teeth. 

 The left ala of the anterior end of the vomer is chiefly developed 

 in the Halibut and other flat fishes. In the Lepidosteus, the 

 vomer is divided into two, as in Batrachia, by a median cleft. 

 Although its posterior end joins obliquely to the under part of 

 the presphenoid, it is not, therefore, less a continuation of the 

 basicranial series than is the postsphenoid, which joins in a 

 similar manner with the basioccipital. 



The prefrontals defend and support the olfactory prolongations 

 of the cerebral axis, give passage to these so-called ' olfactory 

 nerves,' bound the orbits anteriorly, form the surface of attachment 

 or suspension for the palatine bones, and through these for the 



