so 



University of California. 



[Vol. 3 



The humerus of this form, instead of being more slender 

 than that of the later genera, as one would conjecture, is 

 remarkably short and broad. Measurements of the right and 

 left elements of the specimens of osmonti give the greatest 

 length, taken from a point between the radius and ulna, as 170 

 mm. The greatest transverse diameter across the proximal 

 end is 174 mm., and across the distal extremity approximately 

 180 mm. The least diameter, measured across from a deep, 

 narrow notch on the anterior side to a broader posterior emar- 

 gination, is 136 mm. As nearly as the writer has been able to 

 determine, this is the shortest propodial segment known in the 

 limb of an Ichthyopterygian form. 



The anterior notch, to which attention has just been called, 

 reminds one of those found in the radius of the longipinnate 

 Ichthyosauria, and in other elements belonging to the middle 

 portion of the limbs of both longipinnate and latipinnate forms. 

 Lydekker* suggests that in these cases "were it not for the 

 difficulty of explaining the presence of the same feature in the 

 carpals and tarsals the notch in the anterior border of the 

 radiale, tibiale, and phalangeals, might be regarded as the last 

 trace of the shaft of a "long bone." .... In osmonti 

 this notch is very narrow. In a portion of a much less robust 

 humerus with the specimen of alexandrce (PI. 12) it is much 

 wider. The greatly shortened humerus of Shastasaurus presents 

 almost exactly the conditions which Lydekker supposed necessary 

 for the development of a notch, and its occurrence here tends to 

 strengthen his theory. 



The proximal end of the humerus is very convex, the 

 cartilaginous portion extending far around both anteriorly and 

 posteriorly. It is greatly thickened on the inferior or posterior 

 face by a prominent ridge, extending three-fourths the length of 

 the bone, and giving to this end a thickness of about 85 mm. 

 The prominence is like the proximal end of the pectoral ridge of 

 the Mosasaurian genus Platecarpus. The upper surface imme- 

 diately opposite the proximal end of the inferior ridge shows a 

 considerable excavation, which is bounded by a faint ridge in 

 front and a broad, prominent elevation behind. The ulnar 



* Catalogue of Fossil Reptilia and Amphibia in Brit. Mus. Part ii. p. 70. 



