IOWA ACADEMY OF SCIENCES. 33 



the loess of the Mississippi valley have ascribed its deposition 

 to water in lakes or sluggish streams * 



Some have also contended that this occurred in a glacial 

 climate, or at least in a climate much colder than that of to-day 

 in the same region, f 



That fresh water has been regarded as the agent of deposi- 

 tion is due in no small degree to the belief that a very con- 

 siderable portion of the species and individuals found in the 

 deposit consists of aquatic or semi-aquatic forms, although the 

 fact has long been recognized that terrestrial species prevail. 

 There is, however, absolutely nothing in the loess fauna to 

 indicate that the loess land-surfaces were more moist, or to any 

 extent more widely or more deeply covered with waters, than 

 are the surfaces of iowa and Nebraska to-day, — the evidence, 

 if it suggests any difference, indicating rather less moisture 

 than is found in eastern Iowa at the present time. This state- 

 ment, which cannot be too strongly emphasized, is based upon 

 the study of the modern molluscs of Iowa and Nebraska, and 

 their fossil prototypes, extending over a period of nearly 

 twenty years. 



Every species of molluscs which has thus been reported 

 from Iowa, Nebraska, J and Missouri is living to-day, § and with 

 three or four exceptions all are found living within the territory 

 covered by the loess. The conditions which are offered for 

 comparison are therefore exceedingly favorable. 



The following account of the species of MoUusca which are 

 are found in the loess is given with special reference to 

 the habits of the modern forms. Table I at the close of the 



*rhe foUowing are among the chief references: G. 0. SwaUow, Geol. Sur., Missouri, 

 Vols. I and II, p. U; W. H. Pratt, Proc. Dav. Acad. Sci., Vol. I, p. 97; O. A.. White, Geol. 

 Sur., Iowa, Vol. 1, pp. US and 115; Sir Ohas. Lyell, Prin. of Geol., Vol. I, p. 461; W. J. 

 McGee, Proc. Am. A. A. S., Vol. XXVII, p. 32; Archibald Geikie, Text-book of Geol., p. 

 384 (he adds, however, that the loess shades ofif into rain-wash and a^olian formation); 

 S. Aughey, Sketches of Phys. Geog. and Geol. of Neb., p. 373, etseg; R. D. Salisbury, 

 Ark. Geol. Sur., Vol. II., pp. 335-230; McGee, Rep. U. S. Geol. Sur., Vol. XI, p. 574, etc. 

 More recently Prof. Calvin shows In the Rep. Iowa Geol. Sur., Vol. VII, p. 89, that the 



loess materials are derived from the drift, but says that the nature of the agents 



"Is not so clear." He adds that the piling of loess around the margin of the drift is better 

 explained by currents. 



tSome of the references are as follows, ,T. E. Todd, Proc. Am. A. A. S., Vol. XXVIl 

 p. 6; W.J. McGee, Proc. A. A. A. S., Vol. XXVII, pp. 31-3; W. J. McGee and R. E. Call, 

 On the loess and Associated Deposits of Des Moines, pp. 32-3; R. D. Salisbury. Rep. Ark. 

 Geol. Sur., Vol. II, p. 339; W. J. McGee, U. S. Geol. Sur., Vol. XI, p. 574, etc. 



tNo account is here taken of Professor Aughey 'a long list of southern'' forms from 

 the Nebraska loess, its unreliable character having already been pointed out by'the 

 author in Bull, Lab. Nat. Hist. State Univ. of Iowa, Vol. II, p. 95. 



iZonites Shimekii Pilsbry, the only species thus far accredited to the loess alone, is 

 undoubtedly the widely distributed terrestrial Zonites nitidtia. 



3 Lla. Acad. Sc, Vol. v.] [April 27, 18981 



