MEHKTAM: THIASSir K 'I [Til V( )S.\ I'lJIA. 21 



no detailed study of this portion of the skeleton has been carried out in these 

 specimens, and only the most general observalions can be made on the figures 

 available. Of the other material obtained in Europe, only a few fragments are 

 known. Of the American Upper Triassic fauna the skulls of Shasta.^i iinix, 

 Merriamia and Delphinosaurus are known in part, but without satisfactorily 

 showing the features which ordinarily give most definite expression to varia- 

 tion tendencies. Of the American Middle Triassic Cymbospondylus practical- 

 ly the entire skull structure is well shown in several excellent specimens (pis. 2 

 to 6). As Cymbospondylus is the only Triassic genus of which the skull is well 

 known, its description is made the basis for the following discussion of the skull 

 of the Triassic Ichthyosauria in general. 



Although it represents one of the most ancient known ichthyosaurs and is 

 relatively primitive in many respects, the skull of Cymbospondylus is still typ- 

 ically ichthyosaurian. While its structure, like that of the limbs, approximates 

 that of shore types more closely than do the corresponding features in the Ju- 

 rassic ichthyosaurs, it is as a whole, surprisingly specialized, and is evidently 

 the product of an ancestry which had expressed this special type of aquatic 

 adaptation for a long period. Like the typical Jurassic genera, the facial region 

 is relatively long and slender, the premaxillaries and nasals are very largely 

 developed, the frontals are small or are largely covered by the nasals, the su- 

 perior nares occupy a nearly median position, and a heavy sclerotic ring is pres- 

 ent. On the other hand in the relatively greater size of the maxillaries, smaller 

 orbits, and broader temporal bar, the skull is more primitive than any de- 

 scribed form from a later period. 



The skull as a whole is relatively short compared with the length of the 

 trunk. As nearly as can be estimated its length equals little more than one- 

 third that of the dorsal region. The skull of Mixosaurus seems to be relative- 

 ly much longer, equalling about two-thirds the length of the trunk, while in Ich- 

 thyosaurus its relative length may be as great as 80 per cent. 



Facial Region. The antorbital portion of the skull (fig. 3) is elongated and 

 narrow as in the typical Ichthyosauria. It is, however, shorter relative to the 

 total length of the skull, than in any other ichthyosaurian known to the writ- 

 er, excepting the short-faced Ichthyosaurus brcviceps from the Lower Lias of 

 England. In Cymbospondylus (no. 9950) it equals about 60 per cent, of 

 the total length of the skull, and in /. brcviceps 55 per cent. At the same time 

 the extent of the maxillary anterior to the orbit is relatively larger than in any 

 other ichthyosaurian. It amounts to 48 per cent, of the length of the antorbital 

 region in specimen 9950, while in 7. breviceps with a somewhat shorter ros- 

 trum, in which one would expect to find the maxillary relatively long, it is less 

 than 40 per cent. In Mixosaurus the length relative to the cranial region is a 

 little less than in Cymbospondylus. 



