LII INTRODUCTORY REMARKS. 



Hallvllle, three miles west of Black Butte, collected by Dr. Hayden in 1870, 

 they were, from their close affinities to European Eocene species of the same 

 genus, referred by the writer to that horizon.* On examining a few other 

 shells, however, from a still lower position in this series, seven or eight miles 

 farther westward, near Point of Rocks, collected by Dr. Hayden during the 

 following summer, he was, on the contrary, led, rather by their general simi- 

 larity to Cretaceous forms than from the belief that they were really identical 

 with any known Cretaceous species, to include them in the Cretaceous list. -f- 

 These belong to the genera Ostrea and Anomia, the latter being new and 

 peculiar, while the former was very doubtfully referred to a California Cre- 

 taceous species, the provisional name O. Wyomingensis being suggested for 

 it in case it should be ascertained to be a new species, as was believed most 

 probably to be the case. At this lime, both the geographical and stratigraph- 

 ical relations of the localities and beds from which these fossils were collected 

 at the two localities were unknown to the author. 



On visiting this region in 1873, he learned for the first time that 

 the Hallville and Point of Pock localities, which had afforded the fossils 

 mentioned, were only a few miles apart, and that the strata between the 

 two horizons, as well as far above that of the Hallville mines, up to and 

 including the beds at Black Butte station, where the Saurian and other fossils 

 were found, contained forms indicating deposition in brackish- or possibly in 

 some cases salt-water, and that lignite beds occur at various horizons 

 through the whole. The fossils collected at this time belong to the genera 

 Ostrea, Anomia, Corbula, Modiola, Corbicula, Goniobasis (an American 

 type of Melanian), a shell similar to Viviparus trochiformis of the Upper 

 Missouri Port Union Lignite group, and Corbicula cythriformis, of the Judith 

 River group. 



The last-mentioned fossils, and species of the other genera, were found, 

 as already slatea, almost directly associated with the Dinosaurian bones and 

 Tertiary types of plants mentioned above. Of the shell so nearly resembling 

 Viviparus trochiformis, only a few very imperfect specimens, consisting of the 

 upper turns of the spire, were found; but in form and surface-markings they 

 agree almost exactly with the corresponding part of V. trochiformis, which led 

 to the expression of the opinion that they might possibly belong to the same 

 species, though they were not definitely referred to it. 



* l>r. Haydeii's Secoud Ann. Re(>. Geol. Survey of the Territories, 298, 314. 

 t I>r. Ilimlrn's Fifth Ann. Rep. Geol. Survey of the Territories, 375. 



