fiO MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



show more uniformity in the character of the coracoid than is seen in the ichthy- 

 osaurian groups represented in the Jurassic. There is, however, a general ten- 

 dency toward narrowing of the proximal portion much as is common in the 

 forms of the Lower Lias. 



The scapula in all known Triassic ichthyosaurs is relatively broad distally, 

 and in most of the genera as Mi.rosditnts, Shastasaurus, and Merrinmia the 

 broad distal blade with an anterior hook shows considerable resemblance to 

 the form seen in the Mosasauria. In Delphinosaurm the same form of distal 

 expansion is seen, but the proximal region of the anterior margin is not cut 



81 82 83 



Fig. 81. Mixosaurus(^) atavus (Quenstedt). Intrrclavicle, X %. (After von Huene.) 

 Fig. 82. Mixosaurus cornalianus (Bassani). Intorclavicle, X Mi- (After von Huene.) 

 Fig. 83. Ichthyosaurus quadriscissus Quenstedt. Interclavicle, X >. (After von Huene.) 



out so as to produce an anterior hook. In Gymbospondylus the form of the 

 element is more like that in Ichthyosaurus, though the distal end is very much 

 broader and the proximal region of the anterior side is much enlarged. 



The clavicle of Cymbospondylus appears to be considerably more expanded 

 in the middle than in other forms. In Mixosaurus this element is rather 

 slender excepting at the median end. The clavicle of Shdstdsdttrus is slender. 



It is rather remarkable that the interclavicle is not recognized on any 

 American specimen representing the Triassic Ichthyosauria ; excepting pos- 

 sibly the type of Merriamia, in which its form is not clearly shown. In Mi.ro- 

 xdunts it is markedly broader than in the later forms, and might better be 

 denominated triangular than T-shaped as in Ichthyosaurus. Von Huene (1902, 

 p. 11) has described from the Wellenkalk near Calev a thick triangular bone 

 which he recognizes as an ichthyosaurian episternum. Placed in series with 

 the interclavicles of Mixosaurus and Ichthyosaurus quadriscissus, as has been 

 done by von Huene (figs. 81-83), there appears a very marked shrinking of 

 the originally triangular element to the three radii of the triangle in Iclill///- 

 osaurus. 



The shoulder girdle of the earliest known representatives in the Triassic was 

 already considerably specialized in accommodation to the necessary functions 

 of the fairly large paddles. The degree of specialization seen in this arch is 

 probably greater than in the pelvis, inasmuch as the anterior limbs being 



