628 REPORT UNITED STATES GEOLOGICAL SURVEY. 



fessor Whitfield suggests that the Dun Glen variety may possibly be a" 

 new species." 



Eumicrotis carta was not found associated with Pentacrmus at any 

 point in our district, but at several localities where the section was ob- 

 scure it was found with — 



Aviculopecten Idahocnsis 1 

 Lingula brevirostris. 

 Myalina sp. '? 

 Myacites sp. ? 



The beds, however, were above the Eed Beds, but I was unable to 

 determine their relations to the Pentacriims beds. 



In other areas Eumicrotis curta has been found associated with Jur- 

 assic fossils. In the areas surveyed by the Fortieth Parallel Survey it 

 occurs with — 



Belemnites. 

 Gryphwa. 



In our district, we have seen that it occurs associated with the Triassic? 

 fossils in the " Meekoceras beds" which proves, as Dr. White has said,* 

 a great vertical range for the species. He also remarks that Terebratula 

 augusta (Hall and Whitfield) has been considered by the authors as a 

 Jurassic species. 



The Cephalopods of the " Meekoceras beds," described by Dr. White, 

 are, he says, unquestionably Triassic, according to European standards, 

 and resemble Cephalopods of the Musckelkalk of Europe. 



The only other Triassic Cephalopods from the West are those obtained 

 by the Fortieth. Parallel Survey, from the Star Peak Group in Nevada, 

 and they have been considered the fauna! equivalents of the St. Cassian 

 and Hallstadt beds of the Austrian Alps. 



The Star Peak Group, as we have seen, is referred by King to the 

 Upper Trias, while the fossils from our district appear to indicate Mid- 

 dle Trias as the age of the beds from which they were obtained. The 

 fossiliferous rocks have, however, not been closely enough studied as 

 yet to predicate their exact horizon, and we cannot, therefore, be too 

 cautious in speaking of their age. 



There are numerous circumstances that appear to indicate that in the 

 Jura-Trias, as in other formations, there is what may be called a plane 

 of paleontological indefiniteness at either end of the formation. This 

 has often been stated by Dr. White, and as he has remarked, when the 

 deposition of sediments is continuous from one formation to another, it 

 is not strange that forms of life should continue uninterruptedly from one 

 to the other. It is not strange, therefore, that we should find, as in 

 ISTew Mexico,t Triassic plants at the top of the Jurassic, and, as in our 

 district and at many other localities, Jurassic invertebrates at the base 

 of the Trias, and as in Nevada, Palaeozoic types continuing into the 

 lower Mesozoic. The advisability, therefore, of retaining the name of 

 Jura-Trias for the present is obvious. With the accumulation of data 

 inany of the points that are n< >w obscure will assume the certainty of 

 facts. 



To recapitulate briefly, the investigations of the season of 1877 indi- 

 cate — 



1. The lithological separation of the Jura-Trias of Southeastern Idaho 



* Bulletin U. S. Geograph. and Geol. Survey, Vol. V, No. 1, p. — . 

 t See Report U. S. Geol. Survey for 1875-1876, pp. 84-87. 



