24 MEMOIRS OP THE UNIVERSITY OF CALIFORNIA. 



In Mixosaurus and Shastasaurus the orbits are relatively large, though per- 

 haps somewhat smaller than in Ichthyosaurus. 



The sclerotic ring of Cymbospondylus (pi. 5, fig. 2) has much the same struc- 

 ture as in Ichthyosaurus and Baptanodon. The number of plates is about six- 

 teen to eighteen. In Ichthyosaurus the number ranges from fifteen to twenty. 

 Gilmore finds but fourteen plates in Baptanodon, and the same number is given 

 for O ptlialmosaurus by Andrews. The inner or median ends of the plates bend 

 quite sharply over the equatorial region of the eyeball and extend for some dis- 

 tance over the inner side of the eye. The lateral flattening of the eyeball shown 

 by the sharp curvature of the plates on the equator of the eye has been noted also 

 for Ichthyosaurus by Owen 9 and recently on Baptanodon by Gilmore. 10 The 

 relative size of the sclerotic ring agrees with that of the orbit in being smaller 

 than in any of the later ichthyosaurs. The ring in Ichthyosaurus platyodon and 

 iu the short-headed /. breviceps is nearly half again as large as in Cymbospon- 

 dylus. 



Temporal Region. The lateral temporal bar of Cymbospondylus is charac- 

 terized by uncommon length anteroposteriorly. Its height is somewhat less than 

 in most ichthyosaurs. Relative to the length of the cranial region it is much 

 less elevated than in Baptanodon. Behind the jugal the inferior side of the 

 temporal bar is sharply cut out so as to form with the slightly concave coro- 

 noid region of the mandible an elliptical opening with its long diameter ex- 

 tended anteroposteriorly. According to the suggestion of McGregor 11 the tem- 

 poral bar originally contained a lateral temporal opening, which was closed 

 through gradual anteroposterior compression of the bar owing to the enlarge- 

 ment of the orbit. In Cymbospondylus the bar shows fully as much area on the 

 side of the skull as in many reptilian forms possessing a lateral temporal f en- 

 estra. As the quadratojugal passes above the inferior notch in the bar, it is 

 improbable that this opening corresponds to a lateral temporal fenestra which 

 was being lost in Cymbospondylus through breaking down of an inferior tem- 

 poral arcade. Though there is no doubt that the temporal bar has been pro- 

 gressively reduced, it seems very doubtful whether it ever enclosed a lower or 

 lateral temporal fenestra. 



Occipital Region. The occipital region of Cymbospondylus (fig. 6) seems to 

 be well enclosed with bone and the posterior temporal openings are small. The 

 basioccipital differs from that of other ichthyosaurians in presenting a strongly 

 concave articular face for contact with the convex anterior face of the atlas. 

 The inferior side of the basioccipital is characterized by the development of 

 prominent hypapophyses similar to those of Sphenodon. The form of the lat- 



Owen, E., Fossil Rept. Lias Form., pt. 3 (Monog. Pal. Soe., Lond., 1881), p. 103. 



10 Gilmore, C. W., Mem. Carneg. Mus., vol. 2, p. 328. 



11 McGregor, J. H., Mem. Amer. Mus. Nat. Hist., vol. 9, p. 91. 



