S6 MARINE REPTILES OE THE OXFORD CLAT. 



opening is usually situated nearer to the anterior angle of the lachrymals than to their 

 orbital border, which is thickened and concave. 



The prefrontals (pr.f.) are small and form the supero-anterior portion of the border 

 of the orbit, which, however, they do not overhang, as in the Metriorhynchida?. 

 Internally they join the nasals in front in a convex suture, and for a short distance 

 posteriorly they unite with the frontals ; externally they join the lachrymals in 

 a nearly straight suture. In some cases there is a trace of the presence of a 

 downwardly projecting process from the ventral face of the prefrontal ; this, no doubt, 

 is the process which, in Mystriosaurus and in modern Crocodiles, runs down to join 

 the upper surface of the palatine and helps to define the orbit antero-internally. 



The frontals (f.) are, as usual in the group, represented by a single bone. This 

 terminates anteriorly in a point which is thrust between the nasals to a degree 

 varying in the different species, so that the form of the anterior portion of the frontals 

 usually supplies important diagnostic characters. Behind their union with the nasals, 

 the frontals widen out and for a short distance join the prefrontals. Behind this 

 again they curve outwards and backwards, forming the postero-superior portion of the 

 border of the orbit. Posteriorly, the united bones are produced into three processes, 

 a pair running outwards and, in some cases, a little backwards, and joining the post- 

 frontals in an oblique suture, thus completing the bars of bone separating the orbits 

 from the supratemporal fossa?. The third process is a median one, which joins the 

 parietals and forms the anterior portion of the narrow crest dividing the two supra- 

 temporal fossa? from one another ; the summit of this bar is flattened or rounded. 

 In the angle between the median and lateral bars the frontals also help to form the 

 skull-roof. The upper surface of the body of the frontals is gently concave from side 

 to side and, even if the rest of the surface of the skull is unsculptured, usually 

 bears an ornament of rounded pits, which may be very strongly or very slightly 

 developed. Probably on this point there is considerable individual, as well as specific, 

 variation. 



The parietals {par.) unite in front with the frontals and form the greater part of the 

 narrow sagittal ridge between the supratemporal fossa? : the sides of this ridge are 

 vertical and a little concave from above downwards. Below the parietals widen out 

 to help to form the cranial roof, but in no case could sutures between them and the 

 surrounding bones be well seen ; it appears probable, however, that the cranial part of 

 the parietals unite with two bones, the anterior being the alisphenoid, the posterior the 

 prootic. Behind the hinder end of the sagittal ridge the upper surface of the parietals 

 forms a triangular area which may bear a sculpture of pits. Ventrally the united bones 

 join in suture the upper surface of the supraoccipital ; the line of union seems to be 

 marked by a groove, at each end of which there may be a perfoiation. The lateral 

 processes of the parietals join the squamosals to form the posterior wall of the supra- 

 temporal fossa ; the outer end of the lateral ramus of the parietals seems to form part 



