April 4, 1913] 



SCIENCE 



503 



do not materially affect the truth of this 

 statement, for these may be mere races, or 

 a more critical study of these difficult 

 genera may show that the appar-^utly ex- 

 tinct forms are living to-day. They are 

 certainly very closely related to existing 

 forms, and suggest no peculiar set of con- 

 ditions. 



Because of the practical identity of the 

 fossil and certain modern molluscan faunas 

 important conclusions may be drawn con- 

 cerning the conditions which existed at the 

 time of the deposition of the fossiliferous 

 deposits. 



The first question which naturally comes 

 to the mind of the student of Pleistocene 

 geology in this connection is this : Could 

 these mollusks have lived under glacial or 

 near-glacial conditions? As in so many 

 other cases, the measure of the past must 

 here be sought in the present. Unfortu- 

 nately, we do not anywhere have conditions 

 which exactly parallel those which existed 

 in the interior of our continent during the 

 several advances of the ice-sheets. The 

 climate of Alaska is so materially affected 

 by warm ocean currents that no fair basis 

 for comparison can here be established. 

 The Antarctic region presents no partly 

 glaciated continents, and comparisons are 

 here impossible. Perhaps Greenland offers 

 the nearest parallel, but even here in the 

 study of plant and animal life we have to 

 deal with a narrow coastal strip the climate 

 of which is manifestly affected by the 

 proximity of the sea. 



From this narrow strip Moller reported, 

 in 1842, four species of terrestrial mollusks 

 (a Vitrea, a Pupa, a Vitrina and a 8uc- 

 cinea) and three species of pond snails, 

 two of the genus Lymnaa and one of 

 Planorhis. These species were all repre- 

 sented by few widely scattered individuals, 

 and constitute a very scant fauna. No- 

 where in all the barren ground belt border- 



ing the Arctic ice have such combinations 

 of species been observed as we find in the 

 various Pleistocene deposits of our coun- 

 try. The latter suggest rather a region 

 very similar to the present northern part 

 of our country, in the main, with relation- 

 ship in some of the deposits with the fauna 

 of the southern states, and in others some 

 affinity with that of the coniferous belt to 

 the north. 



Our present knowledge of the habits and 

 distribution of the fossil and modern mol- 

 lusks forces the conclusion that the deposits 

 containing these species are interglacial or 

 post-glacial, and that they were formed 

 during a period of mild climate. 



The second important question concerns 

 the immediate conditions under which the 

 various deposits were formed. To answer 

 this question intelligently on the basis of 

 the molluscan fauna we must understand 

 the habits of the mollusks as well as the 

 conditions under which the land and fresh- 

 water forms may mingle. 



For purposes of comparison your essay- 

 ist has made extensive studies of the habits 

 of the modern moUuscan fauna of the Mis- 

 sissippi Valley, and has compared large 

 series of shells thrown up along the shores 

 of larger streams and their smaller tribu- 

 taries, as well as ponds and lakes, with the 

 shells obtained from various Pleistocene 

 deposits and from more modem alluvium, 

 and nowhere has he found any evidence 

 that the conditions under which the Pleis- 

 tocene deposits were formed were ma- 

 terially different from those which are in 

 operation in the same region to-day. 



All of the Pleistocene deposits, with the 

 exception of the loesses and certain buried 

 sand dunes, are aquatic, and a comparison 

 of their molluscan contents shows the same 

 peculiarities and the same variations as are 

 presented by the modern alluvial and 

 water-drifted fauna. 



