8 PALEONTOLOGY OF THE EUEEKA DISTRICT. 



of species, and still further increase the number of species common to the 

 eastern and central (or Atlantic and Mississippi) areas and the western or 

 Rocky Mountain area. 



The fauna of the Upper Carboniferous limestone is composed of old and 

 well-known species usually occurring at that horizon, and gives but three 

 species new to the region of the Rocky Mountains, viz, Ptikdictya carbonaria, 

 P. serrata, and Macrodon tenuistriata. The comparatively few species occur- 

 ring in the middle and upper portions of the Lower Carboniferous Group 

 are also well-known forms, but at the lower horizon we meet with a most 

 interesting assemblage of species. It embraces a large number of Lamelli- 

 branchiate shells, a class so rarely represented in collections from this 

 region, and unites the characters of the fauna of the Lower Carboniferous 

 groups of the Mississippi Valley with that of the Coal-Measures in a remark- 

 able degree, a feature not uncommon in the Lower Carboniferous of the 

 Rocky Mountains, but rarely so well shown as in the Eureka District. 



There is also a certain commingling of Upper Devonian species with 

 the Lower Carboniferous fauna. We find Discina Newberryi, Macrodon 

 Hamiltonce, Grammysia Hannibalensis, G. arcuata, Sanguinolites JEolus, and 

 Pleurotomaria nodomarginata associated with common Carboniferous species. 



The discovery of Pulmoniferous mollusks of the genera Physa and 

 Zaptychius in association with the fresh-water shell AmpuUaria^ Powelli and 

 fragments of a flora, coniferous in character, supports the stratigraphic 

 evidence of the presence of a near or not distant land area at the time of 

 the deposition of the Lower Carboniferous rocks of Central Nevada. It 

 also gives the first notice of the occurrence of the Pulmonifera in rocks of 

 this age; the land shells of Nova Scotia and Illinois occur in the Coal- 

 Measures, and Strophites grandceva, Dawson, is from the Devonian plant 

 beds of New Brunswick. The bearing of this discovery on the question 

 of the presence of land areas from the time of the Middle Paleozoic to the 

 present is important, No other explanation offers than that there was a con- 

 tinuous fresh-water habitat, ponds or streams, which permitted the genera 

 to descend in a direct line from Paleozoic time to the present. 



The grouping of the genera and species in the strata is shown in a 

 general manner in the systematic list at the end of this volume, and in 



