146 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



A specimen (no. 9614; pis. 15 and 18) obtained from the Upper Triassic of 

 Shasta County, California, in 1903 shows several characters of limbs and ver- 

 tebrae not previously seen in any of the material from this region. It re] re- 

 sents a large animal with greatly abbreviated propodial and epipodial limb 

 elements. The vertebral centra are unusually short anteroposteriorly, while 

 the short neural spines are nearly round in cross-section, and are much swollen 

 at the distal end. 



The characters of the limb elements (pi. 15) resemble somewhat those in 

 S. osmonti. The humerus is much shorter and broader than in that species 

 and is altogether the most abbreviated proximal limb segment known in any 

 ichthyosaurian. The bone is also remarkably thick throughout, although the 

 pectoral ridge shows relatively but little elevation compared with that of S. 

 osmonti. The anterior edge is sharply emarginated as in the other species. 

 On the distal end the faces of articulation are not distinctly separated by an 

 angle as in S. osmonti, and the surface is considerably excavated. The radius 

 is relatively large compared with the humerus. Both its length and width are 

 greater compared with the corresponding dimensions of the humerus than in 

 the other species. Its length is also less compared with its own width. Ex- 

 cepting the wide but rather sharply marked notch on the anterior margin, the 

 outlines of the bone are gently rounded, in contrast to the somewhat ang- 

 ular form of the radius in S. osmonti. The posterior margin of the bone is 

 convex instead of concave as in all other known Triassic ichthyosaurs, indi- 

 cating the absence of a distinct gap between radius and ulna. Excepting the 

 notched anterior side the whole border of the radius is sharply excavated by 

 a continuous groove running around the margin. 



MEASUREMENTS OF LIMB ELEMENTS, SPECIMEN 9614. 



Humerus, axial diameter 156 nun. 



Humerus, greatest transverse diameter 212 



Humerus, greatest thickness of proximal end 138 



Humerus, greatest thickness of distal end 90 



Radius, axial diameter ]2(> 



Radius, greatest transverse diameter 156 



In specimen 9614, ten complete vertebrae are present in series, with the 

 upper arches and ribs very little disturbed. The centra are large and are re- 

 markably thin anteroposteriorly (pi. 18, fig. 5<t). Judging from the position 

 of the diapophyses they belong to the middle dorsal region. The centra are 

 thinner than in any other species of Sliastasaunis, unless it be S. core/ji. The 

 diapophyses are long and narrow. The anterior and posterior faces of the 

 centra are evenly but deeply biconcave. The neural arches are relatively short, 



