Vol. 52.] OF THE PLESI08AURIAN SKULL. 247 



The palatal surface of the basisphenoid (b.sph.) rises abruptly 

 from the basioccipital ; it is slightly concave from side to side, 

 and is sharply separated from the lateral surfaces, which make an 

 angle of from 100° to 120° with it. The posterior portion of these 

 lateral surfaces forms a facet, looking outward and downward, with 

 which the pterygoid articulates. The basisphenoid seems to have 

 been overlapped by aparasphenoid (pas.), but the hinder border 

 of that bone is indistinguishable ; anteriorly it expands into a thin, 

 spearhead- shaped plate, the outer angles of which in the present 

 specimen overlap the ventral surface of the pterygoids, and with, 

 them limit the posterior palatine foramina (ptost.pal.vac.), 1 which, 

 open between the basis cranii and the pterygoids, as in Peloneustes. 

 In this latter, however, the parasphenoid is slightly overlapped on 

 its ventral surface by the pterygoids ; this difference in the relative 

 position of the bones in the two genera may be due to displacement 

 in the present specimen. 



The pterygoids (pt.) are triradiate bones, like those of Pelo- 

 neustes, but differ from them in not meeting in the median line 

 over the basisphenoid, and remaining separated by the whole palatal 

 width of that bone. Anteriorly they have been dislocated from their 

 junctions with one another and the surrounding bones, but there 

 can be no doubt that in their natural position they met anteriorly 

 and, together with the parasphenoid, closed the palate in the middle 

 line. 



Their anterior rami are thin triangular plates, the apices of which 

 meet the vomers, while their inner borders form a median suture 

 with one another in front, and are overlapped by the parasphenoid 

 behind. In the uncrushed skull their outer edges united with the 

 palatines. 



The lateral rami run outward opposite the anterior end of the 

 posterior palatine foramina ; their outer ends are much thickened 

 and in the present specimen have been partly broken away. In the 

 skull of P. dolrchodeirus noticed below (fig. 1, p. 248), the outer ends 

 of these lateral rami are joined to the maxillary region by a transverse 

 bone (trs.), and the same is the case in Peloneustes and Pliosaurus. 



In front the posterior rami are narrow bars of bone forming the 

 outer border of the posterior palatine foramina. Behind these 

 openings they widen a little, and bear on their inner side facets foi 

 articulation with the corresponding surfaces on the sides of the 

 basisphenoid. Posteriorly they run outward and backward as 

 thin vertical plates to the quadrates, which do not appear to send 

 forward to meet them plates of bone such as are seen in Sphenodon. 



The columella cranii (PI. IX. fig. 4, col.) or e pi pterygoid is well 



1 In this paper, as well as in that on the skull of Peloneustes (Ann. Mag. 

 Nat. Hist. ser. 6, vol. xvi. 1895, p. 242), the term 'post- palatine foramen ' is used 

 in a different sense from that in which it is sometimes employed (for example, by 

 E. T. Newton in his papers on the Reptilia of the Elo;in Sandstones), pnd is 

 applied to the pair of foramina which result from the division of the median 

 interpterygoid foramen by the basisphenoid and parasphenoid. Newton employs 

 the term for the aperture which lies in front of the transpalatine, and is hero 

 called the ' suborbital foramen.' 



