SKULL 



239 



frontal behind to the prefrontal in front. The eye is surrounded 

 by an inner ring of circumorbital bones. Two large ventral gular 

 plates are present, but no lateral gulars. The operculum is 

 supported by a large opercular and what is probably a subopercular. 

 Unfortunately the cheek region is still incompletely known, and 

 the course of the opercular sensory canal remains to be determined. 

 In Ctenodus (Fig. 209) the paired frontals and parietals are 

 smaller, and a median plate appears between them. This plate 

 becomes larger in Dipterns, where the frontals and parietals are 



Is 



Fio. L>08. 



Restoration of the head of riuineropleuron. co, circumorbital ; /, frontal ; 7*, pterotic(?) ; 

 in.s, median supratemporal (occipital) ; op, opercular ; pa, parietal ; pf, posterior supraorbital ; 

 prf, prefrontal ; pt, small lateral plate ; ptf, postfrontal ; fto, supraorbital. The course of the 

 lateral-line canals is indicated by a dotted line on the right side. 



smaller (Fig. 210). The superficial cranial bones of Dipteru* 

 become more numerous and often strangely irregular, suggesting a 

 secondary subdivision of the lateral plates, somewhat as in the 

 Sturgeons among the Actinopterygii. Ccratodus has diverged in a 

 different direction (Figs. 206-7) : the parietals and frontals are no 

 longer recognisable as such ; the anterior median bone (ethmoid) 

 meets the large posterior occipital (fronto-parietal) ; the supra- 

 orbital and lateral series of bones seem to be represented by 

 two large 'lateral' plates (dermal -lateral ethmoid of Bridge), 

 outside which lie the postfrontal and the * squamosal.' The latter 

 bone, with the ' lateral ' and the occipital, forms a roof over the 



