MEKKIA.M: TKIASSIC ICIITII YOSAUKIA. 143 



bral centra are relatively a little longer than in that species. The coracoid, 

 scapula and humerus are all much broader and thinner than the corresponding 

 bones in S. osinonti. Though the two species are closely related, the differ- 

 ences are of sufficient magnitude to deserve recognition in an attempt to ex- 

 press in a classification the amount of variation in this genus. 



The type specimen is the only individual of atypical Shastasaurus of which 

 any considerable portion of the skull is known. Most of the elements are 

 shown in this specimen excepting the rostral region. The cervical vertebral 

 series is complete, and the elements which are of importance in determining 

 the nature of the change in the rib articulation from double to single have 

 been almost perfectly 'preserved. 



SHASTASAURUS PACIFICUS Mcn-iain. 



Shastagaurus pacificus Merriain, Ainer. Jonr. Sci., 1895, vol. 4, p. 56, figs. 1 and 2. Univ. 

 Calif. Publ. Geol., vol. 3, p. 102, pi. 14. figs. 1 and 2, text-fig. 1. 



The type specimen of this species, constituting also the type of the genus 

 Shastasaurua, was unfortunately represented by only a few posterior dorsal 

 vertebrae and ribs, with one complete pubic bone and a portion of another. 

 Though a considerable quantity of ichthyosaurian material has been known 

 from the Upper Triassic of America, no other specimens have been discovered 

 which seemed certainly to represent this specific type. 



The species seems to be characterized by greater length of the vertebral 

 centra anteroposteriorly than in any other species of 

 Shastasaurus. The neural arches of the posterior dorsals 

 are short and are considerably thickened but do not show 

 the characters of S. altispinus or S. careyi. 



The form of the pubis (fig. 148) is most nearly ap- 

 proached in S. osmonti. It differs from the form in that 

 species in that the proximal end is broader and the pos- 

 terior portion curves forward and inward toward the 

 prominent posterior hook. In S. osmonti the proximal lg ' c */taJ 

 end of the pubis is narrower and is not upturned. about ^ natural size - 



Associated with the type specimen of S. pacificus are three large discoidal 

 elements, similar to those with the posterior limb of specimen 9608, referred 

 to S. osmonti. They appear to have been slightly concave on the upper and 

 lower surfaces. The discs are quite thick, and the edges are concave with an 

 uneven or pitted surface. The form of these elements indicates that they were 

 separated by a considerable body of cartilage. The discs resemble the pha- 



