MERRLUF: THIASSIC H'l FTIIYOSArKIA. 



2.1 



The iiasuJtt (fig. 4) are large and extend hack over the frontals as in other 

 Ichthyosanria. They are relatively very widely expanded, much as in Ichthyo- 

 Mt/triiN and B<t/>t<i>io<l</. Xo comparison with Mixo*<u<niK has been made, as 

 the writer has not been able to make certain of the form of this element in that 

 genus. From Repossi's figures of the skull (Repossi, 1902, tav. 9, fig. 1) one 

 suspects that the nasals are very large. 



Sp 



Fig. 5. Opthalmosaurus icenicus Seeley. Skull. (Adapted from 0. W. Andrews.) About % natural 

 size. Pm, premaxillary ; M, maxillary; N, nasal; L, lachrymal; J, jugal; Pf, pref rental; Ptf, 

 postf rental; Pto, postorbital; Pr, parietal; Sq, squamosal; St, supratemporal ; Qj, quadratoju- 

 gal; Q, quadrate; S, stapes; Bo, basioceipital ; Ar, articular; Sa, surangular; An, angular; Sp, 

 splenial; V, dentary. 



Fronto-Parietcil Region. As in Ichthyosaurus the anterior portion of the 

 cranial region is covered by the expanded nasals. A little more of the frontal 

 element seems to be exposed than often occurs in Ichthyosaurus. The large 

 pineal foramen is surrounded mainly by the parietals, instead of wholly or to 

 a large extent by the frontals as in Ichthyosaurus. 



Orbital Region. One of the most striking features of the skull in Cymbos- 

 pondylns is the relatively small size of the orbit, compared with the orbital re- 

 gion of most species of Ichthyosaurus, or more particularly with Opthalmosau- 

 rus (fig. 5). Even in specimens crushed laterally so that the height of the orbit 

 would tend to be increased, the relative size is practically the same as in the 

 figure above (fig. 3). (See also pi. 6.) In Baptanodon as figured by Gilmore 8 

 the length of the orbit is only a little less than one-fourth that of the whole skull, 

 including the much elongated snout. In Gymbospondylus it is about one-sixth 

 the length, in spite of the fact that the rostral region is relatively short. The 

 relative size of the orbit is particularly noticeable in comparison with the an- 

 teroposterior diameter of the temporal bar immediately behind it. The antero- 

 posterior diameter of the temporal bar equals about 38 per cent, of the antero- 

 posterior diameter of the orbit in Cymbospondylus (no. 9950) ; in Baptanodon 

 it appears to be less than 25 per cent. Of the undistorted Ichthyosaurus skulls 

 of which the writer is able to obtain any information, the forms which most 

 nearly approach Cymbospondylus in these characters are /. platyodon and I. 

 lonchiodon of the English Lower Lias. 



8 Gilmore, C. W., Mem. Carneg. Mus., vol. 2, pi. 7. 



