THE AFTONIAN MOLLUSKS 121 



the more accurate determination of the climatic conditions which pre- 

 vailed during the Aftonian interglacial period. 



Some light is thrown upon these conditions by the presence of a mam- 

 malian fauna made up of large herbivorous animals, which probably re- 

 quired great quantities of plant food and could live only in a climate 

 supporting abundant terrestrial plant life. While all these species of 

 mammals are now extinct and it is not entirely safe to draw conclusions 

 concerning habits from mere relationship, it seems reasonable to conclude 

 that the climate was sufficiently mild to support the necessary plants. 



Significance of the Aftonian molluscan Fauna 

 the aftonian mollusks 



The molluscan fauna, though apparently insignificant when compared 

 with the giant mammals which were discussed by Calvin, furnishes evi- 

 dence concerning climatic conditions which is really much more con- 

 clusive, for the mollusks of the Aftonian are identical with those which 

 inhabit Iowa and adjacent territory today, and the habits of all these 

 species are definitely known. 



But the very fact that these species exist today introduces an element of 

 possible error which makes the exact determination of the horizon from 

 which they come very necessary; for similar or identical series of shells 

 may be collected both in our present waters and in very modern alluvial 

 deposits, and these species probably invaded the region of Iowa during 

 every interglacial period of Pleistocene time. 



The following species of mollusks have been collected by the writer 

 from the Aftonian beds in Iowa : 



Aquatic Species 



TJnio metanever Raf. Ancylus rivularis Say. 



Vnio anod 071 1 aides Lea (?). Lymncea reflexa Say. 



*Sph(Brium sulcatum (Lam.) Pr. *Lymnwa caperata Say. 



*Pisidimn , probably 2 species. *Lymna:a humilis Say. 



*Amnicola . *Physa . 



*Bythinella obtusa (Lea) St. *Planor'bis bicarinatus Say. 



*Valvata tricarinata Say. *Planor'bis parvus Say. 



Valvata bicarinata Lea. *Planorbis dilatatus Gld. 



Terrestrial Species 



Polygyra (fragment). Zonitoides arboreus (Say) Pils. 



Pyramidnla alternata (Say) Pils. Bifldaria armifera (Say) Sterki. 



Pyramidula striatella (Anth.) Pils. Succinea ovalis Say (=:obliqua Say). 



^Vallonia gracilicosta Reinh. *Succinea avara Say. 



*Yitrea hammonis (Strom.) Pils. Succinea retusa Lea. 



