Merriam.] 



Triassic I chili yopterygia . 



85 



outside by the overlapping supra- angular. Toward the posterior 

 end of both rami the angular is seen resting below the supra- 

 angular. The sutures between these two elements are fairly 

 well defined along the greater part of their contact, but are 

 unfortunately obscured at several points. On the broken pos- 

 terior portion of the right ramus their is visible the cross- 

 section of a small bone, which may represent the coronoid. No 

 trace of it is seen on the inner side of the left ramus; it is, 

 however, possible that the angular has slipped over it, or that it 

 has been destroyed in preparation. Concerning the form of the 

 articular little can be said, as the region in which it should occur 

 is very poorly preserved. Judging from the form of this part 

 of the ramus, the articular might lie somewhat different from 

 that described for Ichthyosaurus, but this point must be settled 

 by the study of better material. 



Dentition. — In a recent note (Merriam, 1902) the writer stated 

 that the dentition of Shastasaurus is differentiated. This infor- 

 mation was based on the study of a small skull, supposed to belong- 

 to this genus. Later preparation of this specimen by the use of 

 acids has made a more careful study of it possible, and it is found 

 to differ so much from the typical Ichthyosaurs, and apparently 

 also from Shastasaurus, that its reference to this group appears 

 at present unwarranted. 



A number of loose, conical teeth, with striated enamel 

 crowns, have been collected in the limestones in which Shasta- 

 saurus remains occur, but as yet none have been found with 

 recognizable specimens of that genus. Similar teeth occur in 

 these beds in small jaws belonging to other groups, so that while 

 some of them quite probably belong to Shastasaurus we cannot 

 refer any of them to it with certainty. 



AFFINITIES AND CLASSIFICATION. 



In the outlines of its structure, Shastasaurus shows itself 

 clearly a member of the Ichthyopterygia. The skull, vertebra^, 

 tail fin, extremities, and pectoral arch are of the type of those in 

 Ichthyosaurus, and quite unlike the general aspect of the skeleton 

 in any other group. In some of its characters, particularly the 

 rib articulation in the dorsal region, the fusion of the ha?ma- 



