362 LIFE OF THE PLEISTOCENE 



an excellent measure of the conditions which prevailed at the time of the deposi- 

 tion of the various fossil-bearing Pleistocene strata. 



"The terrestrial mollusks which are found in the Pleistocene deposits are 

 also now of very wide distribution and the variation which they exhibit in spe- 

 cies, form and size is not at all determined by latitude, but rather by the eda- 

 phic conditions under which the forms existed. In both cases the species are 

 those of modern faunas whose habits are well known. 



"Variations in the Pleistocene fauna are nowhere better illustrated than in 

 the loess, which has a wide distribution both north and south, and east and 

 west, in the Mississippi Valley. If we begin in the northwestern part of the 

 loess area in Nebraska and western Iowa, we find that the dominant species in 

 the loess are Pupa muscorum, P. blandi, Pyramidula shimekii, Succinea gros- 

 venori, Oreohelix iowensis, Vallonia gracilicosta, Bifidaria procera, Sphyradium 

 edentulum alticola. These species all belong to a fauna characteristic of the 

 dry western regions, Pupa muscorum alone passing by a wide detour northward 

 to the northeastern part of the country. Other species belonging to more east- 

 erly faunas appear, as a rule, in smaller numbers. Southward along the 

 Missouri River, as in northwestern Missouri, larger forms, such as Circinaria 

 concava, Pyramidula alteruata and Polygyra multilineata, more characteristic of 

 eastern and southeastern faunas, begin to appear in larger numbers. The 

 change southward along the Mississippi is even more striking. In the north- 

 erly deposits along the Mississippi Helicina occulta, Pyramidula striatella, 

 Succinia ovalis and S. avara are among the most common species. Pupa decora 

 is also abundant in both northern and northwestern loess, and while it is largely 

 a boreal species, like Pupa muscorum, it extends along the western mountains 

 well into our dry western region. 



"Southward along the Mississippi the loess molluscan fauna changes in 

 essentially the same manner as the modern fauna of the surface. At Hickman, 

 Kentucky, the larger helices (so prominent in the southeastern modern fauna) 

 already appear in large numbers and Pyramidula solitaria, carinate Pyramidula 

 alteruata, Polygyra tridentata, very large P. albolabris, large P. profunda, a few 

 P. elevata, P. fraterna, P. fraudulenta, P. appressa, Omphalina fuliginosa, large 

 Circinaria concava, more abundant Pyramidula perspectiva and Gastrodonta 

 ligera. These species already form the most conspicuous feature of the loess 

 fauna. Helicina occulta still appears, though here approaching its southern 

 limit. Still farther south at Dyersburg, Tennessee, a similar fauna appears 

 in the loess, but Helicina occulta is not common, reaching here its southern 

 limit and Pyramidula striatella, so common in the north, also becomes rare. 

 Still farther south on the west side of the Mississippi River at Helena, Arkan- 

 sas, the loess fauna becomes still more characteristically southern, and in addi- 

 tion to the larger helices already mentioned the large form of Succinea ovalis, 

 Omphalina kopnodes, Vitrea placentula and Helicina orbiculata appear in 



