346 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 30 Ser. 



Whitney collection, and the collection of the United States 

 National Museum, embracing the collections made by the 

 Hayden Survey in southeastern Idaho, and by the Survey 

 of the Fortieth Parallel in Nevada. He has also studied all 

 the private collections where Triassic fossils were to be 

 found. A part of the results of this work will shortly 

 appear in a paper by Alpheus Hyatt and James Perrin 

 Smith, "The Triassic Cephalopod Genera of America," 

 to be published in the Professional Papers of the United 

 States Geological Survey. In this work every genus of 

 cephalopods known to occur in the Trias of America is 

 described, and a representative species under each is de- 

 scribed and figured. 



Geography of the Trias in America. During Triassic 

 time the sea, which had covered the greater part of the 

 Mississippi Valley and the Great Basin in the Upper Car- 

 boniferous, had retreated westward until it was reduced to 

 a mere gulf, of which the approximate outlines are shown 

 on plate xl. It is not likely that the sea covered all the 

 area indicated on this map during the whole of the Trias, 

 nor at any one time. Sediments with marine fossils of the 

 Lower Trias are known on the North American continent 

 only in eastern California and southeastern Idaho; marine 

 fossils of the Middle Trias are known only in California 

 and central Nevada; while Upper Triassic marine fossils 

 are known only in northern California, central Nevada, 

 western British Columbia, on Queen Charlotte and Van- 

 couver islands, and on the shores of Alaska. 



During the Lower Trias the gulf extended as far east- 

 ward as the Aspen Mountains of Idaho; during the Middle 

 Trias it retreated westward until its eastern border was in 

 central Nevada; and at the end of the Trias the land had 

 encroached still further until the gulf was little more than 

 a bay in northern California, and central Nevada, with 

 similar bays in western British Columbia. 



Around this western gulf extended the inlets and con- 

 tinental basins in which were deposited the Triassic 



