OPHTHALMOSAUEUS. 



the anterior wall of which is formed by the upper surface of two prominent bosses, 

 separated by a deep notch and each terminating in a surface for cartilage : these are 

 the processes called by Siebenrock *, in his account of the skull of Hatteria, the lower 

 cylindiical processes [v.cp.) ; they mark the beginning of the presphenoidal region of 

 the basis cranii, which remained unossified. Beneath them is the posterior end of the 

 parasphenoid (pas.), which is adherent to the ventral face of tliis bone, as described 

 below. The hollow bounded by the upper surface of the lower cylindrical processes 

 below, and by the posterior clinoid process behind, is the pituitary fossa {pit.fos.), into 

 which, as already noted, the very wide carotid canal opens and passes downwards 



Text-fiff. 5. 



v.c.fi. fias. ^5^ 



Basisphenoid of OpJitJialmosawus : A, from below ; B, from the front. (R. 2162, | iiat. size.) 

 h.c, foramen for a branch of the carotid artery; i.c.f., internal carotid foramen; jjfw., parasphenoid; 

 jo.cl., posterior clinoid processes ; pit.foss., pituitary fossa ; p^/., facet for pterj-goid ; pt.2}., pterygoid 

 processes ; v.cp., lower cylindrical processes of basisphenoid. 



and backwards, its single ventral aperture being situated about the middle of the 

 ventral face of the bone : this lower opening is usually more or less asymmetrical in 

 form. 



In some of the Liassic Ichthyosaurs, as pointed out by Cuvier f and more fully by 

 Maggi ;];, tliere are two posterior openings, separated, in some cases at least, by the 

 posterior end of the parasphenoid (see text-fig. 6, C) ; this condition seems to be 

 the most primitive, since in Ilaiteria the paired openings are situated on either side of 

 the posterior end of the parasphenoid. In Ophtlialmosaurus the condition of the 

 posterior end of the parasphenoid (see below) is very different in diflerent individuals 



* " Zur Osteologie des Hatteria-Kopfes," Sitzungsber. k. Akad. Wiss. Wien., math.-naturw. CL, vol. cii. 

 (1893) p. 250 ; also translated in Ann. Mag. "N"at. Hist. [6] vol. xiii. (1894) p. 297. 



t ' Ossements fossiles,' vol. v. pt. 2 (1824) p. 460, pi. xxix. figs. 12 & 13. 



% " II Canale Craneo-faringeo negli Ittiosauri etc.," Rendiconti E. Istit. Lombardo, [2] vol. xxxi, 

 (1898) p. 761. 



