PELOKEUSTES. 41 



the inner border. Looked at from the inner (ventral) side (text-fig. 13) the frontals 

 are seen to extend back to a point bebind the anterior end of the pineal foramen, but 

 are completely excluded from it by the wedge-like prolongation of the parietals 

 which extends between them for some distance in advance of the opening. In 

 front of this ventral prolongation of the parietals, the frontals appear to meet 

 in the middle line for some distance and extend considerably beneath the facial 

 processes of the premaxillae. On their ventral side near the outer edges they 

 are produced downwards into thin lamina?, which curve inwards so as just to meet 

 in the middle line, but without uniting with one another, thus partly enclosing a kind 

 of median tunnel (o.c.) which probably lodged the olfactory nerves. Posteriorly this 

 channel is continuous with the groove leading to the pineal foramen (p.f.)\ anteriorly 

 the enclosing ridges gradually become lower and disappear at about the level of the 

 front of the orbit ; probably they are borne entirely on the frontals. The outer side of 

 the wall of the tunnel just described forms the inner face of the upper part of the 

 orbit, and from it a rounded ridge runs out on to the prefrontals and is continued down 

 to the palatine, forming a partial division between the orbit and the olfactory cavity. 

 Except that in Peloneustes the ridges forming the sides of the olfactory canal are much 

 more strongly developed, the arrangement of the lower face of this part of the skull 

 seems to be similar in a general way to that of Hatteria*. The extension of the 

 parietals in front of the pineal foramen to the premaxillae noticed above, has been 

 described by Williston in Brachauclicnius f and Lolichorhyncops J, two genera of 

 Plesiosauria from the Cretaceous of North America. Possibly in these also it will be 

 found that the frontals do extend back to the level of the pineal foramen and meet in 

 the middle line for at least a short distance, though this union is concealed on the 

 outer surface of the skull by the overlapping parietals. If this is so, the difference 

 between the skulls described by Williston and those of the European Elasmosaurs is 

 less marked than would appear from the descriptions. 



The postfrontal (po.f.) unites mainly with the outer edge of the hinder end of the 

 frontal, but it also has a short contact with the parietal opposite the middle of the 

 pineal opening. Its inner end is broad and, looked at from above, is in part concealed 

 in front by the overlapping of the frontals ; at its outer end it narrows to a simple flat 

 bar of bone, the lower end of which no doubt joined the postorbital to form the division 

 between the temporal fossa and the orbit. 



The prefrontal (pr.f., text-fig. 13) is a large bone which unites on its inner side 

 with the frontal in a long curved suture. At its posterior end it is separated from the 



* Siebenrock, " Zur Osteologie des Hatteria-K.o-p£es," Sitzungsb. math.-naturwiss. CI. k. Akad. Wiss. "Wien, 

 vol. cii. (1893) p. 250. 



t Williston, " The Skull of Brachauchenius," Proc. U.S. Nat. Mus. vol. xxxii. (1907) p. 477. 



J Williston, "North American Plesiosaurs, Pt. I." — Eield Columbian Museum, Geological Series, vol. ii. 

 no. 1 (1903), p. 14. 



PAET II. Q 



