MUR^NOSAUEUS. , 81 



the middle line (PL III. fig. 1) or may be squarely truncated; in the former case 

 the appearance of the presence of a median foramen is produced, and there may 

 indeed be a small nutritive foramen at this point, but, as already described, the 

 paired internal carotids enter the pituitary fossa by lateral foramina, not by a median 

 ventral opening as in Oplithcdmosaurns (see p. 13, fig. 5). The anterior end of 

 the basisphenoid beneath and in front of the pituitary fossa terminates in a pair 

 of blunt truncated processes [v.c.]}., the lower cylindrical processes of Siebenrock) ; 

 these probably joined the cartilaginous presphenoid region. 



The paras})henoid {pas., PI. 111. figs. 1,1a; text-tig. 44) is a thin flat bone, the 

 hinder part of which is closely united with the ventral face of the basisphenoid, 

 though the line of junction usually remains distinctly visible. In front of this it 

 divides the two openings which in former papers* I have called the posterior 

 palatine vacuities (parasphenoidal vacuities of Williston), which are enclosed 

 externally by the pterygoids. Anteriorly the parasphenoid widens out somewhat, 

 and, judging from what occurs in the closely allied genus Trichidus, the outer edges 

 were overlapped for some distance by the pterygoids. In front of this union the 

 bone is abruptly truncated, its border forming the hinder limit of the median 

 interpterygoid vacuity. This form of parasphenoid differs widely from that described 

 by "Williston f in Trinacromerum, where it is compressed laterally, and is so deep 

 posteriorly that its upper surface would seem to be on a level with the cranial 

 surface of the basisphenoid and basioccipital, there apparently being no pituitary 

 fossa. Moreover, according to Williston, the pterygoids meet in median suture above 

 the hinder end of the parasphenoid, so that its posterior end unites with them and 

 not with the ventral face of the basisphenoid as usual. 



The exoGcipital {ejco., PI. III. fig. 1 a ; text-figs. 44 & 45, C, D, E) in all cases observed 

 is fused with the opisthotic, though on the inner face of some specimens the line 

 of junction of the two elements is quite distinct, as it is also in some instances on 

 the articular ends (see dotted line in text-fig. 45, E). The bone formed by the united 

 elements is columnar in form, and the exoccipital portion forms the sides of the 

 foramen magnum. The lower end expands somewhat and terminates in an oval, 

 flattened, more or less roughened facet for union with the basioccipital (Ijoc.f.). The 

 anterior angle of this surface is borne by the lower end of the opisthotic, which 

 is here marked off from the exoccipital in many cases by a groove {oj>.g.), and on 

 the inner side by a distinct notch. The inner face of the combined bones is concave 

 from above downwards, and is perforated by a number of openings. Of these the 



* See figures of skulls of Plesiosaurus in Quart. Journ. Geol. See. vol. lii. (1S96) p. t^.'Sljfig. 2, and pi. ix. 

 fig. 1 ; of Fliosaurus, loc. cit. vol. liii. (1897) pi. xii. ; of Peloncmtes in Ann. Mag. IS'at. Hist. [6] vol. svi. 

 (1895) p. 245, and pi. xili. fig. 1. 



t Williston, " Korth American Plesiosaurs : Trinacromerum,'' Journal of Geologv, vol. xvi. (190S) p. 71S, 

 fig. 5. 



M 



