Vol. l] Merriam. — A Primitive Ichthyosaurian Limb. 



35 



The scapula is very strongly curved laterally to accommodate 

 itself to the curvature of the body. The anterior and posterior 

 borders are broken somewhat and may have extended out into a 

 hook on each side, as in most of the Triassic forms. The outer 

 border is practically entire and is nearly straight, giving the 

 element somewhat more of the form seen in the true Ichthyo- 

 saurs. 



The clavicle is very large and heavy ; much broader and more 

 robust than in Mixosaurus. The median end appears to have a 

 large facet for the inter clavicle, but, strangely enough, that ele- 

 ment has never yet been clearly seen in any of the numerous 

 Ichthyosaurians obtained from the West- American Triassic. 



Taken as a whole, the pectoral arch resembles that of Delphi- 

 nosaurus more than it does that of any other form. As will 

 appear later, these forms also show evidences of affinity in the 

 structure of the extremities. 



As much of the pectoral limb as is known shows more resem- 

 blance to the type seen in Mixosaurus than to any other form. 

 The humerus is of much the same type as in Mixosaurus, but is 

 if anything a little more slender. Of the two humeri figured by 

 Kepossi* the principal illustration (fig. 2) represents an 

 extraordinarily long and narrow form. In the other specimen 

 (fig. 1), as in still others, the humerus is almost as broad as it 

 is long.f 



The epipodial elements are relatively long, as in all of the ear- 

 lier Ichthyosaurs, and the space between them is very broad. The 

 radius is, so far as the writer is aware, the narrowest specimen 

 known in the Ichthyosauria, and the constriction of the median 

 or shaft portion is pronounced. The ulna shows the most primi- 

 tive form known in this group, and at the same time presents 

 certain rather peculiar characters. In all other Ichthyosaurs. 

 excepting Delphinosaurus, the posterior border of the ulna is 

 expanded and usually convex. In these two forms it is concave 

 or notched, like the outer or anterior border of the radius ; and 



*Atti Soe. Ital. Sc. Nat., Vol. 41, PL 9, figs. 1 and 2. 



tit would appear to the writer that Eepossi's figure 4, Plate 9, loe. cit. 

 represents a femur rather than a humerus. It has the form of the femur 

 in other specimens and does not resemble any other described mixosaurian 

 humerus. 



