ACTINOPTEBYGII. 



105 



;ill respects a typical member of the family. The same genus 

 ranges throughout the Jurassic period, and fine specimens occur 

 in the Lithographic Stone of Bavaria (fig. 74). As the genera 



FIG. 74. 



Mesodon macroptenis ; restoration, with cheek-plates removed, about two-thirds 

 nat. size. U. Jurassic (Lithographic Stone) ; Bavaria, fr., frontal ; 

 m.eth., mesethmoid ; md., mandible, showing narrow dentary in front ; 

 op., operculum ; orb., orbit; p.op., preoperculum ; pa., parietal; pmx., 

 premaxilla; s.occ., supraoccipital ; sq., squamosal ; r, vomer. The caudal 

 region is destitute of scales. 



are traced upwards in the rocks they become specialized chiefly 

 (i.) in the strengthening of the backbone by the interlocking of 

 its arches, and (ii.) in the reduction of the tritoral teeth on the 

 vomer and splenial to a fixed arrangement in regular longitu- 

 dinal series. In two rare genera from the Upper Cretaceous of 

 Mount Lebanon, Coccodus and Xenopholis, dermal spines are 

 also developed on the clavicle and cranial roof. The more 



