MKHKIAM: THIASSIC ICHTHYOSATJBIA. 11 



tcetli was also found to differ from that in Ichthyosaurus, inasmuch as the 

 fangs were set into the jaw instead of vesting in an open groove. The sep- 

 arate zygapophysial facets of the vertebrae were shown to be placed in in- 

 tersecting planes, in contrast to the facets in Ichthyosaurus, where they arc 

 brought into the same plane and united medially. The vertebral centra were 

 found to be deeply biconcave, and to be perforated by the chorda dorsalis in the 

 caudal region. The specimens referred to M. at a v us were divided into two 

 varieties, major and minor. 



In 1895 Merriam described the genus Shastasaurus from the Upper Triassic 

 of northern California. The type specimen consisting of eight connected verte- 

 brae, an arch bone and several ribs was distinguished from other genera by the 

 thickened neural spines of the vertebrae, the single-headed dorsal ribs and the 

 peculiar form of the coracoid. The element held to be a coracoid by the writer, 

 and by all others who have expressed an opinion as to its character, was found 

 later to be a pubis quite different from any known ichthyosaurian element. 

 The vertebrae and ribs were from the posterior dorsal region. 



In 1902 the description of the Calif ornian Triassic ichthyosaurs was contin- 

 ued by Merriam with better material available. From these specimens it was 

 possible to determine the principal characters of the skeleton, excepting the an- 

 terior portion of the skull and the phalangeal region of the paddles. The ma- 

 terial was also found to represent several specific forms, one of which, Shasta- 

 saurus perrini, was later removed as a distinct genus, Delphinosaurus. In this 

 publication Leidy's types of the Middle Triassic ichthyosaurs of Nevada were 

 redescribed, and attention was called to their similarity to the shastasaurian 

 forms. A collection of ichthyosaurian material obtained from the Middle 

 Triassic of Nevada by the University of California before this paper ap- 

 peared was referred to as apparently representing Cymbospondylus petrinus 

 of Leidy. This material was stated to show a number of peculiar characters 

 not recognized in specimens previously examined. 



In November 1902 a fully illustrated paper by Repossi furnished the first 

 definite information regarding the general skeletal structure of Mixosaurus. 

 In Repossi 's figures it was shown that Mixosaurus possessed many primitive 

 characters that had not been recognized before. It appeared also to be 

 much more closely related to the American Triassic forms than had been sup- 

 posed. Also in 1902, there appeared Yakowlew's publication on a new find of 

 ichthyosaurian remains from Spitzbergen, in which the large Spitzbergen 

 form (Ichthyosaurus polaris of Hulke) was shown to have single-headed dorsal 

 ribs, and was referred to Shastasaurus. 



In von Huene's valuable work on the Triassic Reptilia, issued in 1902, there 

 was given a brief review of the literature on the Ichthyosauria. Von Huene 

 also presented the first figures of the interclavicle of a Triassic form, showing 



