AM 10 IDE I 



539 



the snout covered with numerous plates ; two large frontals partly 

 separated by a median bone ; irregular small bones representing the 

 parietals, separated in the mid-line by a large occipital plate ; several 

 small supratemporals ; a single bone representing the pterotic and post- 

 frontal ; the orbit limited below by small plates which extend over the 

 sub- and preorbital region. There appears to have been a small oper- 

 cular and a large preopercular extending over the cheek, an exceptional 

 thing amongst Actinopterygians ; no sub- or interoperculum. Only two 





Pig. 319. 



Mesodon maeropterus, Ag. ; Upper Jurassic, Bavaria; restored. (After A. S. Woodward, 

 Vert, Palaeontology.) fr, frontal ; mcth, mesethmoid ; md, mandible, showing narrow dentary in 

 front; op, opercular; orb, orbit; pop, preopercular; pa, parietal; pmx, premaxilla ; ■■■■•, 

 supraoccipital ; sq, pterotic (squamosal) ; v, vomer. The caudal region is destitute of seal— . 



branchiostegal rays remain, and the place of the median gular is taken 

 by a mosaic of small plates. The gill-opening is small. The jaws and 

 palate are much specialised to bear a grinding dentition. The maxilla 

 is slender and toothless. Sharp prehensile teeth occur on the small 

 premaxilla and dentary ; the slender toothless palato - quadrate arch 

 becomes fixed for a considerable length to the basis cranii. On the 

 splenials and opposing fused vomers are longitudinal rows of blunt 

 enlarged grinding teeth. As specialisation increases the rows become 

 fewer and more regular, and the teeth larger (Fig. 317). 



