IC MARINE EEPTILES OF THE OXFORD CLAY. 



In one case (fig. 6, A) it extends back round the sides of the carotid foramen [i.e./.], 

 forming the anterior and part of the lateral border of that opening ; in most (text- 

 fig. 5, A) it extends just to the anterior border of the foramen, while in one instance 

 it only extends about halfway to it (text-fig. 6, B). 



In front of the basisphenoid the parasphenoid (text-fig. 7) extends forwards as a long 

 pointed rostrum, the length of which is about three times that of the basisphenoid in 

 the mid-ventral line. Posteriorly this rostrum is transversely oval in section, but soon 

 becomes triangular, the upper surface being concave from side to side, so that it forms 

 a shallow groove {prs.g.) which received the loAver edge of the presphenoidal bar of 

 cartilage, which probably, as in Hatteria, formed tlie ventral edge of the interorbital 

 septum : this groove occupies about the posterior half of the free portion of the bone. 

 In front of this the rostrum becomes more compressed laterally, and on its sides are 

 long surfaces {pt.f.) slightly concave and ridged, apparently for union with the inner 

 side of the anterior limbs of the pterygoids, between which its anterior half extended. 



'J"he squomosal (text-fig. 8, A, B.) is a bone of complex shape. It occupies the postero- 



superior angle of the skull, where it forms a prominent boss (p.e.a.). From this 



angle there runs forwards a broad plate, the somewhat thickened upper border of 



which forms the posterior half of the outer border (o.b.) of the supratemporal fossa 



(s.t.foss.) ; the ventral and anterior edges of this plate are thin and no doubt united 



with the supratemporal and postfrontal bones, as they are shown to do in Gilmore's 



figure * of the skull of BapTanodon, and as is the case in the earlier Ichthyosaurs. 



From the posterior angle again there runs inwards and a little forwards a very stout 



bar of bone, making an acute angle with that just described and forming the outer 



part of the posterior border of the supratemporal fossa. At its inner end this process 



widens out considerably from above downwards and terminates abruptly in a deeply 



hollowed, somewhat diamond-shaped cavity (pa.f.) for the reception of the outer end 



of the squamosal process of the piirietal. On the posterior face of this process of the 



squamosal there is a small shelf-like projection, making nearly a right angle with 



the quadrate region about to be described : in this angle is the facet for the reception 



of the outer end of tlie opisthotic [op.f.). Beneath the posterior angle (p.e.a.) there 



is a broad plate of bone continuous in front and behind with the processes already 



described: this is the quadrate region of the squamosal (?/".) ; it consists of an outer 



and an inner plate separated by a deep narrow fossa, into which the upper end of the 



quadrate is firmly fixed. The inner plate extends down the inner face of the 



quadrate, to which it is closely adherent, and at its lower end it overlaps the 



ascending quadrate plate of the pterygoid ; by this aiTangement the quadrate, apart 



from its other supports, the stapes and quadrato-jugal, is held rigidly by the squamosal 



above and the pterygoid below. The above description agrees in the main with that 



given by Gilmore in his account of the skull of Baptanodon, but it cannot be said 



* " Osteology of Baptanodon (Marsh)," Mem. Carnegie Museum, vol. ii. (1905) pi. viii. 



