72 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



the third of digit two are so small that the total number could hardly have 

 been larger than in Merriamia. The posterior paddle in this genus (fig. 98) 

 is also much like that in Merriamia, and the presence of a rudimentary fourth 

 or posterior digit is indicated by small facets on the posterior margins of the 

 elements of the third digit. In Toretocnemus the posterior limb is as large as 

 the anterior one. 



Delphinosaunat and Shastasaurus. In both Dcli>lnnos<inrnH and SJidstasau- 

 rus the propodial and epipodial limb elements are shortened as compared with 

 the limb of Merriamia. In Delphinosaurus (fig. 93) the humerus is scarcely 

 longer than broad, but retains a distinctly contracted shaft region. In Shas- 

 tasaurus (figs. 94 and 95) the humerus may be so greatly abbreviated that its 

 width considerably exceeds its length. 



In Delphinosaurus the radius and ulna are of nearly equal size and the ulna 

 differs from that of all other ichthyosaurs excepting Oymbospondylus in 

 showing a posterior notch and a median constriction similar to that of the 

 radius. In Shastasaurus the greatly shortened radius has a narrow notch on 

 the anterior border while the posterior side is either only very slightly con- 

 cave or is distinctly convex as in S. careiji. In that species is found the only 

 exception to the rule that in the Triassic Ichthyosauria the radius and ulna 

 are separated by a distinct cleft. In Shastasaurus osmonti the posterior border 

 of the ulna is convex. 



Although the phalangeal region of the limbs of Delphinosaurus and Shasta- 

 saurus has not been seen in its natural arrangement in connection with the re- 

 mainder of the limb, there seems to be a strong reason for believing that it 

 was of a reduced and presumably tridactyl type as in Merriamia and Tore- 

 tocnemus. In Delphinosaurus where the radius and ulna are of nearly equal 

 size, there are three elements each in the first and second rows of the meso- 

 podial region as in Merriamia, but the posterior elements in both rows are 

 much smaller than the others and much smaller than in Merriamia, suggest- 

 ing that the posterior digit was relatively small. In Shastasaurus the radius 

 is much larger than the ulna. The radiale, the only mesopodial which has been 

 found in place, is considerably larger than in any. of the other genera, and even 

 exceeds the ulna in size. Evidently in this form the space for the posterior 

 digits was considerably narrower than in Delphinosaurus, which suggests that 

 Shastasaurus may have had only two well developed digits and a rudimentary 

 third. 



Though no series of paddle bones showing the form and arrangement of 

 the elements in the podial region of Shastasaiirus has as yet been discovered, 

 in several cases a number of scattered bones representing mesopodials or pha- 

 langes have been found with other parts of the skeleton, but it has not been 

 possible in any instance to make certain of their position in the paddle. With 



