18 



MEMOIRS OF THE UNIVERSITY OP CALIFORNIA. 



In the Upper Triassic specimens, excepting in a very few instances, the lime- 

 stone matrix is cemented to the bones, making the work of preparation more 

 than ordinarily difficult. In the case of small specimens, etching with hydro- 

 chloric acid has been found advantageous ; the blue tint of the mineral viviante 

 appearing in the bones, making their outlines comparatively distinct. 



The Middle Triassic ichthyosaurs of the Pacific Coast region have thus far 

 been known only from the limestones of the West Humboldt and New Pass 

 ranges of Western and Central Nevada (fig. 2). Search has been made in the 

 Middle Triassic shales of California but without any definite results. 



The most important occurrences are those in the exposures along the east- 

 ern side of the West Humboldt Range. Wherever the limestones are exposed 

 bones may occur. The most important finds have been made at Cottonwood, 

 Buena Vista, Straight, Indian, American, and Fisher caiions. In the south 

 fork of American Canon on the two points known as Saurian Hill and Fossil 

 Hill (pi. 1, fig. 1 ) skeletons were particularly numerous and well preserved. 



The horizon at which the ichthyosaurs of the West Humboldt Range occur is 

 apparently a constant one, representing the middle portion of the Middle Tri- 

 assic. The associated invertebrate fauna as described by James Perrin Smith 

 includes with others the following characteristic forms : 



Daonella dubia Gabb. 

 OrtJioceras dubium Gabb. 

 Atractites conf. crassirostris Hauer. 

 Ceratites trinodosus Mojsisovics. 

 Anolcites americanus Mojsisovics. 

 Acrochordiceras hyatti Meek. 

 Gymnites aff. bosnensis Hauer. 



Ptychites meeki Hyatt and Smith. 

 Celtites Italli Mojsisovics. 

 Monophyllites billingsianus Gabb. 

 Sageceras gabbi Mojsisovics. 

 Eutomoceras dunni Smith. 

 Joannites gabbi Meek. 

 Beyrichites rotelliformis Meek. 



This fauna is that of the characteristic Ceratites trinodosus horizon, and G. 

 trinodosus is a common species. 



In addition to the Ichthyosauria there have been found in these beds a few 

 fishes, and some reptilian forms which are as yet only imperfectly known. The 

 following are the described forms." 



Acrodus alexandrae Wemple. 

 Acrodus oredontus Wemple. 



Hybodus nevadensis Wemple. 

 Omplialosaurus nevadanus Merriam. 



The skeletons are in general better preserved in the Middle Triassic beds of 

 Nevada than in the later limestones of northern California, and are more easily 

 freed from the matrix. The limestone is often slightly shaly, and is frequently 

 separated from the bone by a thin film of argillaceous material heavily irnpreg- 



Wemple, Miss E. M., New Ce'straciont Teeth from the West-American Trias. Univ. Calif. Publ. Geol., vol. 5, 

 no. 4. 



Merriam, J. C., Preliminary Note on a New Marine Reptile from the Middle Triassic of Nevada Univ 

 Calif. Publ. Geol., vol. 5, no. 5. 



