MKRKIAM: TRIASSK' IC1ITII V( ISAI'IMA. 7:', 



specimen 9081 a number of disroidal bones are associated with the elements of 

 the shoulder girdle and the proximal portion of the limb. Three of these not 

 far from the radius appear from their position to represent a transverse row 

 from the proximal portion of the paddle (Cr. fig. 94). At one end of the row 

 the ossicle is much smaller than either the median one or the one at the op- 

 posite end. While it is not possible to make sure whether the elements seen 

 here belong in the podial or in the mesopodial region, the evident reduction 

 of one of the terminal bones supports the evidence for reduction of the third 

 or posterior digit as suggested above. 



In several of the specimens of Sliastasaurus discoidal ossicles present with 

 the limb elements are deeply pitted or grooved around the margins (fig. 113; 



112 "3 114 



Fig. 112. CymbospondyllU pctrinus Leidy. Metapodials or phalanges, X l /- 



Fig. 113. Shaxtasawits osmonti Mcrri.-un. Metapodial or phalange, marginal view, X %. 



l''ig. 114. Baptanodon sp. Phalange, marginal view, X%. 



also pi. 15, figs. 3 and 4). This structure, with the circular outline developed, 

 evidently indicates that they were embedded in heavy cartilage plates as in 

 Baptanodon (fig. 114). The only instance in which other than discoidal 

 elements have seemed to represent the distal portion of the paddle in Shasta- 

 sditnis is in the type specimen of S. altispinus (pi. 18, fig. 4), in which there 

 are two ellipsoidal bones sharply notched at one end, but with the margins 

 grooved and pitted. The notches evidently indicate that the bones were on the 

 margins of the paddle. 



In both Delphinosaurus and Shastasaurus the femur is more slender than 

 the corresponding elements of the anterior limbs. (See figs. 100 and 101.) The 

 tibia is relatively large compared with the small fibula. Both tibia and fibula 

 show a median constriction. Of the elements beyond the epipodials only a few 

 discoidal ossicles are known. 



STAGE OF EVOLUTION OF TRIASSIC ICHTHYOSAURS. 



In viewing the Ichthyosauria collectively there seems to be no reason for 

 questioning the unity of the group. Throughout the entire series of forms in 

 which the history of the order is represented there is no doubt as to relation- 

 ship ; the oldest and the youngest members showing so strong a resemblance to 

 the typical genus that no question as to their common descent has arisen. In 



