Yol. 52.] OF THE PLESIOSATJRIAN SKULL. 249 



The lower surface of the maxillae and premaxillce is largely 

 concealed by the mandible, which is tightly closed upon them. The 

 inuer border of the palatal plate of the maxilla is, however, visible 

 for some distance both in front of and behind the internal nares, the 

 outer border of which it forms. In its anterior region there are 

 one or two pits which probably mark the points of eruption of 

 successional teeth. The palatal portion of the premaxilke is almost 

 completely concealed by the symphysial region of the mandible ; 

 but the anterior ends of the vomers appear to run forward some 

 distance between these bones : in the above-mentioned skull of 

 P. dolichodeirus this is certainly the case (fig. 1, p. 248). 



The general structure of the Plesiosaurian palate is shown dia- 

 grammatically in fig. 2 (p. 251). 



The structure of the temporal arcade (PL IX. fig. 3) is, in 

 all essential respects, similar to that in Plesiosaurus brachycephalus 

 (figured by Sollas), 1 P. dolichodeirus, and P. Hawkinsi (figured by 

 Owen), 2 and also to that of Peloneustes 3 : the only important difference 

 being that in the present species the postorbital sends back a long thin 

 strip along the anterior ramus of the squamosal nearly to its origin. 

 The supra-jugal which Sollas observed in P. brachycephalus cannot 

 be detected, but, if I understand the description of that bone (it is not 

 figured), it corresponds to the lower portion of the postorbital. The 

 thin posterior extension of the maxilla along the lower edge of the 

 jugal is concealed by the mandible, the pressure of which has driven 

 it inwards. 



The wall of bone described by Sollas, which separates the orbit 

 from the temporal fossa, is well shown in this specimen. It appears 

 to be mainly formed by the postfrontal and postorbital, each of 

 which thus consists of an external facial and an internal postorbital 

 portion, which meet in the angle forming the anterior rim of the 

 temporal fossa. I cannot make out what share in the formation of 

 this postorbital wall is taken by the jugal; according to Sollas it is 

 an important one. 



The upper ramus of the triradiate ' squamosal ' is in this specimen 

 indistiuguishably fused with the remainder of that bone ; but in the 

 younger skull described by Owen it is separated by a distinct suture, 

 which is figured by him. 4 He calls this upper portion the ' mastoid,' 

 while the remainder of the bone, consisting of the inferior and 

 anterior rami, is designated the 'squamosal.' It is clear that these 

 two elements are equivalent to the supra-temporal and squamosal of 

 lizards, according to the terminology of Parker & Pettany and 

 many other writers, or to the squamosal and prosquamosal, according 

 to Paur. Their arrangement is similar to that occurring in the 

 Ilhynchocephalia, the fused elements of the older individuals having 

 almoso exactly the form and relations of the so-called ' squamosal ' 



1 Quart. Journ. Geol. Soc. vol. xxxvii. (1881) pi. xxiv. fig. 2. 



2 Trans. Geol. Soc. ser. 2, vol. v. pt. iii. (184U) pi. xlv. 



3 Ann. Mag. Nat. Hist. ser. 6, vol. xvi. (1895) p. 251, fig. 2. 

 * Trans. Geol. fc>oc. ser. 2, vol. v. pt. iii. (1840) pi. xlv. 



