324 



University of California Publications. 



[Geology 



genus Pessosaurus. If the several specimens referred to this 

 genus actually belong together, the only distinguishing character 

 of the group would seem to be in the humerus. The element con- 

 sidered as the femur is like that of Shastasaurus. The short 

 humerus is nearly circular in form, and differs from that of 

 Shastasaurus in the absence of any trace of a median constric- 

 tion. In the most specialized forms of Shastasaurus the humerus 

 is relatively shorter than in P. polaris, but evidence of an original 

 median constriction appears in the anterior and posterior 

 notches. The carpal and phalangeal elements of Pessosaurus 

 are apparently round, and evidently lay in a cartilage plate as 

 in Shastasaurus and in Baptanodon. 



PESSOPTERYX 



Of the specimens referred to Pessopteryx the only remains 

 of the skull consist of jaw fragments, of which the exact position 

 in the skull is not known. The teeth carried by these fragments 

 are numerous, and the hemispherical crowns seem to be set in 

 many rows. They were first thought by Wiman to be pavement 

 teeth of fishes, but against this idea he has rightfully urged that 

 they are numerous, and are the only teeth occurring with the 

 saurian remains in the lower horizon. The presumption is in 

 favor of the view that they represent the same form as some of 

 the saurian bones. As is suggested by Wiman, the dentition 

 represented in these specimens differs from that of ichthyosaurs 

 in the quite uniformly smooth, hemispherical tooth-crowns, 

 and in their arrangement in numerous rows. In one specimen 

 figured, the roots are shown to have infolded walls, as in the 

 Ichthyosauria. It is, however, worth noting that the figured 

 specimen (Wiman, pi. 9, fig. 30) showing the folded root-struc- 

 ture is much larger than any of the others. The specimen shown 

 in Wiman 's figure 26, plate 9, which most nearly approaches in 

 size the specimen with a slightly folded root-structure, does not 

 certainly show the arrangement of teeth in several rows. The 

 combination of characters seen in this dentition is quite unlike 

 that of any known member of the Ichthyosauria. Very low, 

 domed tooth-crowns are known in the most posterior teeth of the 



