THE DISTRIBUTION OF LOESS FOSSILS 12 J 



preceding forms. On the other hand, at Council Bluffs and 

 Omaha the modern shells of this species are usually small, like 

 those of the loess, though both fossil and modern shells of the 

 large type occasionally occur. Thus the fossils of this species 

 from the eastern part of the state resemble both the fossil and 

 modern shells from the western part. Succinea avara is another 

 example. The small typical form is common in the loess at 

 Iowa City, but the modern shells are not frequent, occurring 

 always on more or less wooded hillsides, while westward the 

 type is the common modern form. 



In the loess of both the east and the west, 1 Sphyradium eden- 

 tulum alticola, Pyramidula strigosa iowensis? Succinea grosve?iorii, 

 forms belonging now to the dry western plains, are quite com- 

 mon. Their presence, together, with that of the "depauperate" 

 forms, when considered in connection with the entire molluscan 

 faunas of the eastern and western parts of the state, suggests a 

 climate considerably drier than that of the eastern part of 

 the state, and a surface less abundantly timbered. Certainly 

 both modern and fossil faunas unmistakably show 3 that the con- 

 ditions in the eastern and western parts of Iowa during the 

 deposition of the loess were approximately included within the 

 bounds of the present extremes presented by these regions, and 

 that any attempt to drag into the discussion of this subject con- 

 ditions either of a glacial climate or of frequent and widespread 

 floods and inundations, or of any excess of moisture, is gratu- 

 itous. 



The conditions which cause the depauperation of our shells 

 exist more or less all over Iowa today, especially westward, 

 and yet we do not have a glacial climate. If the molluscs 



1 The loess herein designated as "eastern" is that of eastern Iowa — the "west- 

 ern " being that of western Iowa and eastern Nebraska. 



2 This form has heretofore been reported as var. cooperi which lives abundantly in 

 the far West, but Pilsbry regarded it as extinct and distinct, and has described it 

 under the name iowensis. All living forms of strigosa belong to the high, dry regions 

 of the West. Neither of these species was found at Council Bluffs, but both are found 

 in the loess of Nebraska. Sphyradium was formerly included in Pupa. 



3 See also the writer's paper in Proc. la. Acad. Sci.,Vol. V.— particularly p. 42. 



