PRELIMINARY REMARKS. 



The species described in the following pages were collected in and 

 around the Black Hills during the expedition, and represent the differ- 

 ent formations recognized, except the Carboniferous, from which the 

 fossils obtained were few and of little interest. 



The horizon of the Potsdam formation of the Black Hills appears to 

 be, so far as the fossils will serve to determine, about the same as that 

 o^f Wisconsin and the neighboring States ; and of some of the layers 

 the lithological features are so nearly the same that it would be difficult 

 to distinguish between specimens from the two localities. The purplish- 

 green quartzitic rock from the head of Bed Canon Creek, containing 

 Lingulepis pinniformis Owen sp. and several other Brachiopodous shells, 

 appears to hold nearly the same relation to the rest of the group as 

 does that at the Falls of the St. Croix, from which the species was orig- 

 inally described, namely, near the base ; the Bed Canon Creek beds 

 resting immediately on the slates of older date (probably Huronian) 

 while the Trilobitic beds and the soft friable layers occur at a horizon 

 of about 100 feet below the Carboniferous. The entire fauna of the 

 Black Hills beds closely resembles that from Wisconsin ; although the 

 species are nearly all distinct, the generic facies is the same through- 

 out. Among the fossils from the higher parts of the formation there 

 are quantities of Plant remains belonging to the genus PaloBopJiycua 

 which, I am informed, cover the surface of the rocks over large areas 

 where exposed, being densely matted together. These beds also hold a 

 position similar to the Plant beds of other localities. 



The Jurassic formations seem also to be well represented ia this region, 

 and many of the beds are highly fossiliferous. The species are not 

 numerous, but the individuals are extremely abundant, many of the 

 slaty calcareous layers being so largely composed of organic remains, 

 mostly Lamellibranchiates, as to be quite friable and easily broken in 

 the hand, and some of the sandy layers have their surfaces densely cov- 

 ered with the separated shells of one or two species. The absence of 

 Gasteropoda is a noticeable feature at all localities from which specimens 

 have been brought, no example of this class of molluscs being present 

 in the entire collections from this formation. The few species of Fresh- 

 water Gasteropoda described, as probably from this horizon, in the 

 Smithsonian Contributions, (Pal. Up. Missouri,) being still doubtfully 

 referred. This same absence of Gasteropoda is also noticed in all the 

 collections of Jurassic age we have seen from the western countries, 



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