144 MEMOIRS OP THE UNIVERSITY OF CALIFORNIA. 



langes of Baptanodon, but fail to show even the tendency to angularity seen 

 in the proximal mesopodials of that form. Inasmuch as elements of this char- 

 acter are also found associated with the epipodial region in the types of S. 

 osmonti and D. perrini, it is possible that they are proximal mesopodials. In 

 D. perrini (no. 9082), the anterior and middle proximal carpals are however 

 slightly angular. It is not improbable that in this species as in other forms of 

 Shastasaurus the mesopodial and phalangeal elements were all reduced to v the 

 peripherally grooved or pitted discoidal form. 



Though the characters of this form of ShaxtdMtiirns approach those of S. 

 osmonti, and it may ultimately be necessary to unite them, no intermediate 

 types are known. As long as the two forms can be distinguished the interests 

 of classification are best subserved by recognizing them as distinct. 



SIIASTASAURUS ALTISPINUS Merriain. 



Shastasaurus altispinus Merriam, Univ. Calif. Puhl. Geol., vol. 3. p. 99, pi. 14. fijz;. 5, and 

 pi. 15. 



Vertebral centra not differing greatly in form and proportions from S. 

 osmonti; neural spines of dorsal vertebrae very thick transversely. Except- 

 ing the blade-like anterior and posterior margins, they are nearly circular in 

 cross-section. Distal end of spines hollowed by a groove-like anteroposterior 

 excavation. Heads of dorsal ribs with two faces sharply turned away from 

 each other; upper face usually more noticeably pitted than lower. The known 

 podial limb elements with rounded and excavated or deeply pitted margins, 

 but with a distinct notch on the outer side. 



Very little is known of this species excepting the type specimen and a 

 small amount of fragmentary material which may belong here. It seems to 

 be fairly distinctly separated from S. osmonti and S. alexandrae with shorter 

 and broader neural spines and less specialized rib heads. In S. altin}>i>nis the 

 upper arches of the middle dorsal vertebrae are more than 20 per cent, higher 

 than the centra, and in S. osmonti they are 10 per cent, shorter than the centra. 



In S. altispinus the peculiar double facetted character of the heads of the 

 dorsal ribs seems to be more strongly expressed than in any of the other species 

 (pi. 18, figs. 3ffl and 37;). The lower face is generally less distinctly pitted than 

 the upper, and was evidently in direct contact with the lower portion of the 

 diapophysis. The upper face may be turned away from the lower face as much 

 as thirty-five degrees. The surface of the upper face is generally quite dis- 

 tinctly concave. As is shown under the general discussion of the rib articu- 

 lation of Triassic Ichthyosauria (p. 52) the adjustment of the rib to the 



