8 PALEONTOLOGY OF THE EUREKA DISTRIOT. 
of speciés, 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, Ptilodictya 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 
Hamiltone, Grammysia Hannibalensis, G. arcuata, Sanguinolites Afolus, 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 Ampullaria? 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 grandeva, 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 ina 
general manner in the systematic list at the end of this volume, and in 
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