April 4, ]913] 



SCIENCE 



507 



as yet been found, and it is evident that 

 the fossiliferous Pleistocene deposits were 

 formed after the interglacial and postgla- 

 cial conditions had been fully established. 



In no known Pleistocene deposit is there 

 a vertical gradation of species which can 

 be accounted for on climatic grounds. 



The variation in horizontal distribution 

 does not in any case indicate a climate of 

 greater severity than that which exists in 

 the same region to-day. The determinable 

 deposits containing iiuviatile and other 

 aquatic shells are of such limited extent 

 north and south, and, moreover, the species 

 which they contain are now so widely dis- 

 tributed that they present no evidence of 

 climatic variation. 



The terrestrial mollusks which are found 

 in the Pleistocene deposits are also now of 

 very wide distribution and the variation 

 which they exhibit in species, form and 

 size is not at all determined by latitude, 

 but rather by the edaphic 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 Mis- 

 sissippi Valley. If we begin in the north- 

 western part of the loess area in Nebraska 

 and western Iowa, we find that the dom- 

 inant species in the loess are Pupa mus- 

 corum, P. hlandi, Pyramidula shinieki, 

 Succinea grosvenorii, Oreohelix ioensis, 

 Vallonia gracilicosta, Bifidaria procera, 

 Sphyradium edentulum alticola. These 

 species all belong to a fauna characteristic 

 of the dry western regions, Pupa mus- 

 coruni alone passing by a wide detour 

 northward to the northeastern part of the 

 country. Other species belonging to more 

 easterly faunas appear, as a rule, in smaller 

 numbers. Southward along the Missouri 



River, as in northwestern Missouri, larger 

 forms, such as Circinaria concava, Pyra- 

 midula alternata and Polygyra midtilin- 

 eata, 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 northerly deposits along the Missis- 

 sippi Helieina ocnlta, Pyramidula striatella, 

 Succinia ovalis and S. avara are among 

 the most common species. Pupa decora is 

 also abundant in both northern and north- 

 western 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 essen- 

 tially the same manner as the modern 

 fauna of the surface. At Hickman, Ken- 

 tucky, the larger helices (so prominent in 

 the southeastern modern fauna) already 

 appear in large numbers and Pyramidida 

 solitaria, carinate Pyramidula alternata, 

 Polygyra tridentata, very large P. albolah- 

 ris, large P. profunda, a few P. elevata, 

 P. fraterna, P. fraudulenta, P. appressa, 

 Omphalina fxdiginosa, large Circinaria 

 concava, more abundant Pyramidida per- 

 spectiva and Gastrodonta ligera. These 

 species already form the most conspicuous 

 feature of the loess fauna. Helieina oc- 

 culta still appears, though here approach- 

 ing its southern limit. Still farther south 

 at Dyersburg, Tennessee, a similar fauna 

 appears in the loess, but Helieina occulta 

 is not common, reaching here its southern 

 limit and Pyramidula striatella, so com- 

 mon in the north, also becomes rare. Still 

 farther south on the west side of the Mis- 

 sissippi River at Helena, Arkansas, the 

 loess fauna becomes still more characteris- 

 tically southern, and in addition to the 

 larger helices already mentioned the large 



