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SCIENCE 



[N. S. Vol. XXXVn. No. 953 



form of Succinia ovalis, Omphalina ko- 

 pnodes, Vitrea placentula and Helicina 

 oriiculata appear in conspicuous numbers. 

 The last three species are distinctively 

 southern. Helicina occulta has wholly dis- 

 appeared and its place has been taken by 

 Helicina orbiculata. The richly fossilif- 

 erous loess of Natchez and Vicksburg, 

 Mississippi, also contains the forms com- 

 mon at Hickman and Helena, and the 

 presence of Polygyra obstricta, P. inflecta 

 and P. stenotrema still further stamps the 

 fauna as distinctively southern. 



But in this variation in the wide loess 

 region there is nothing which suggests a 

 transition or change from cold climate to 

 warm climate faunas or vice versa. The 

 variation, as we find it in the loess is prac- 

 tically exactly duplicated in the modern 

 fauna of the surface. The only conclusion, 

 then, which can be drawn from the fossils 

 of the loess is, that during the deposition 

 of the several loesses climatic conditions 

 were not materially different from those 

 which exist in the various parts of the same 

 general region to-day. Such differences as 

 do exist point rather to a drier climate in 

 the northern part of the loess-covered area 

 than that of to-day. 



Emphasis has sometimes been placed 

 upon the depauperation in size of certain 

 loess shells, as evidence that the climate in 

 which they existed was colder than that 

 of to-day. These depauperate shells are 

 found only in the northern part of the loess 

 area, in Iowa, Nebraska, etc. Their exact 

 counterparts are found living to-day in the 

 drier portions of the same region. And 

 corresponding differences do not occur in 

 more easterly series which represent differ- 

 ences in latitude. It is evident that the 

 depauperation is due to drouth and not to 

 a low temperature, and the abundance of 

 these depauperate shells in the northern 

 • loess reinforces the evidence already noted 



that the climate of this region was then 

 somewhat drier than at present. 



The earlier hasty conclusion in this case 

 illustrates only one of the diiificulties which 

 the student of geology who has not given 

 special attention to the mollusks encoun- 

 ters in his efforts to determine the condi- 

 tions under which various Pleistocene de- 

 posits were formed. 



Another results from erroneous identifi- 

 cations of deposits. An illustration of this 

 is furnished by the Otis Mill section in 

 South Dakota opposite Chatsworth, Iowa, 

 which has been referred to the loess. The 

 section, in fact, consists of Aftonian silt 

 and sand, Kansan drift and true loess. The 

 fossils, to which reference is usually made 

 in connection with this section are of the 

 usual alluvial type and belong to the Af- 

 tonian part of the section only, none being 

 found in the capping loess. Still other 

 errors result from erroneous identifications 

 of species. Thus Garychium exile, the spe- 

 cies usually found on high ground, has 

 been identified as C. exigiium, a species 

 which is frequently amphibious. Other 

 errors arise from the assumption that spe- 

 cies of the same genus have the same habi- 

 tats, and that their presence in various 

 deposits indicates like conditions. This is 

 often far from true, thus, Zonitoides arho- 

 reus and Z. nitidus are similar in appear- 

 ance, and closely related, yet the former is 

 an upland species, while the latter is found 

 in low grounds and is sometimes amphibi- 

 ous. Similarly, Succinia avara is upland, 

 while S. retiisa is amphibious; Pomatiopsis 

 lapidaria is terrestrial, while P. cincinnati- 

 ensis is found only in somewhat deeper 

 water in large ponds and lakos; Planoriis 

 trivolvis lives in shallow waters in ponds 

 and swampy borders, while the similar P. 

 truncatus is found only in the larger lakes 

 in deeper waters. Lymncea palustris and 

 L. emarginata show a similar difference in 



