138 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



As yet no true SJiostasaurus is known from the Middle Triassic formation of 

 Nevada. 



From the evidence of a very small quantity of fragmentary material it ap- 

 pears possible that ichthyosaurian forms at least allied to Shastasawms have 

 had a wide geographic distribution in Triassic times. From an examination 

 of the measurements of Hulke's Jcli/l/yoxu/inix />o/<irix the writer has sug- 

 gested (1902, 2, p. 88) that this form closely resembles in character the pos- 

 terior dorsals of Shastasmtrtts. More recently Yakowlew (1902, 1, p. 194) has 

 described additional material from this region. This he refers to the species 

 polaris, which is considered by him to be a true Skwtasauarus. As is indicated 

 elsewhere (p. 150) this form is possibly nearer to ymbospon<bylns than to 

 Sliastasaurus. 



SIIASTASAURUS OSMONTI Merriam. 

 Shastasaurus osmonti Merriam, Univ. Calif. Publ. Geol., vol. 3, p. 93, pis. 8, 9, 10, and 1 1 . 



Vertebral centra relatively longer than in S. careyi, and somewhat shorter 

 than in S. pacificus or altispinus. Neural arches of dorsal vertebrae showing 

 more lateral compression than S. careyi and altispinus. Coraeoids and hume- 

 rus much broader and thicker than in S. alcxandrae. Humerus less abbreviated 

 than in S. careyi. 



This species was the first American Triassic ichthyosaur of which enough 

 material was found to make possible a satisfactory determination of its syste- 

 matic position. The type specimen consisting of thirty-five vertebrae from the 

 anterior portion of the column, together with the most important elements of 

 the anterior arch and limb, was obtained by Mr. V. C. Osmont from the Atrac- 

 tities Beds of the Hosselkus Limestone, at Smith's Cove, on the west side of 

 Brock Mountain, between Squaw Creek and Pit River, in Shasta County, 

 California. Several other specimens representing about as large a portion of 

 the skeleton as the type, also many isolated bones or small parts of skeletons, 

 have been obtained. The species ranges from well down in the Trachyceras 

 Beds of the lower or more shaly division of the Hosselkus Limestone up into 

 the higher, massive, gray limestone horizon represented in part by the Spiri- 

 ferina Beds. It is most common in the upper part of the lower or more shaly 

 division. 



The species includes individuals probably ranging from 12 to 18 or 20 feet 

 in length. 



In the type specimen, no. 9076, Univ. Calif., Col. Vert. Palae., a series of ver- 

 tebrae shows the characters of the -centra from the axis to the 36th centrum. 

 (See fig. 57, p. 48.) As far back as this series of centra extends the faces of 

 articulation of the diapophyses are confluent above with the articular faces for 



