338 



AM 10 1 DEI 



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 



'/jrnoc. 



p;?ioc. 



Fig. 318. 



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

 A. S. Woodward, Vert. Palaeontologi/.) "ft, 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, supraoccipital 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 



