DESCRIPTION OF THE BEDS 403 



Aquatic Species 



Sphwrium sulcatum ( Lam. ) Prime Segmentina armigera ( Say ) H. & A. Adams 



Pisidium sp.? PlanorUs bicarinatus Say 



Valvata tricarinata Say PlanorUs parvus Say 



Valvata bicarinata Lea Lymnma caperata Say (?) Fragments 



Ancylus rivularis Say Physa sp. ? Fragment 



Terrestrial Species 



Vitrea hammonis (Strom.) Pils. Pyramidula striatella (Anth.) Ins. 



Zonitoides arboreus (Say) Pils. Pyramidula alternata (Say) Pils. 



Vallonia gracilicosta Reinh. Succinea obliqua Say 



Bifidaria armifera (Say) Sterki Succinea avara Say 



The aquatic species belonging to the genera Splicerium, Pisidium, and 

 Ancylus are most common and most widely distributed in the beds of 

 finer sand. All the aquatic species in the list are now found living in 

 the same region in the streams tributary to the Missouri, and in adjoining- 

 ponds. 



The fossil land shells are more local in distribution and fewer in num- 

 ber, only scattered individuals being found. The species are all repre- 

 sented in the modern fauna of the same region. 



It may be worthy of note that while all the species of land shells here 

 listed are also found in the loess, they are there never mingled with fresh- 

 water shells in the same manner. 30 On the other hand, such mingling of 

 fresh water and land shells as is here recorded is common in all the 

 modern alluvial deposits of the same region. The presence of these shells, 

 then, shows that fluviatile conditions prevailed in the immediate area con- 

 cerned, but that land surfaces on which the terrestrial mollusks flour- 

 ished, and from which they were washed by floods, were near by. 



No uniformity marks the relative arrangement of the sands and 

 gravels. Sometimes the finer sands form the uppermost member of the 

 series, as shown in plate 33, figures 1 and 2, and plate 35, figure 1 ; again, 

 the heavy gravels occupy this position, as in the county line exposure 

 shown in plate 34, figure 2, or the sands and gravels are indiscriminately 

 interbedded. These differences simply indicate variations in the force 

 of the ancient currents. 



THICKNESS AND ALTITUDE OF BEDS 



Where undisturbed, the sands and gravels are usually very clean, espe- 

 cially along the tributary valleys, but in the main bluffs of the Missouri 

 valley they are not infrequently mingled and interbedded with silt. 



30 Freshwater shells are exceedingly rare in the loess, and are almost exclusively Pul- 

 monates, and such genera as Sphcerium, Ancylus, etcetera, are unknown. 



