338 



AMIOIDEI 



and the facial region descends very suddenly to the small mouth with its 

 pointed jaws (Fig. 318). There is reason to believe that many of the 



A. 



FIG. 318. 



Mesturus leedsi, A. S. W. Restoration of head, A, from above ; B, from the side. (After 

 A. S. Woodward, Vert. Palaeontology.) ag, angular ; br, branchiostegals ; d, dentary ; eth, 

 ethmoidal plates ; /, frontal ; m, median plate ; mx, maxilla ; op, operculum ; orb, orbit ; pop, 

 preoperculum ; pa, parietal plates ; pmx, premaxilla ; socc, snpraoccipital plate ; st, supra- 

 temporals ; spl, splenial bearing tritoral teeth ; sq, pterotic (squamosal) ; v, vomer bearing 

 tritoral teeth ; x, small plate. 



usual bones have by a process of degeneration been split up into a 

 number of small irregular plates, as in the Acipenseridae. 



Taking Mesturus as an example (A. S. Woodward [502]), we find 



