150 IOWA ACADEMY OP SCIENCE 



are two loesses on Crowley's Ridge, but they are not the two 

 described in former reports. The differences between them in 

 color and texture are probably due to variations in the source of 

 dust supply during the period of formation, possibly to shifting 

 of river courses. 



The inclusion of the lower stratum as loess led Call to exag- 

 gerate the thickness of the loess at Helena. It is much less than 

 ninety feet in thickness. 



The fossils. — Both the true loesses of the ridge contain shells, 

 frequently in large numbers. Both, or either one, may be fossi- 

 liferous in the same section, or both may be without fossils. All 

 the fossils (with the exception of Planorliis (Gyraulus) deflectus, 

 reported by Call as rare, but which the writer failed to find) are 

 terrestrial. Even the operculate Pomatiopsis lapidaria, which 

 Call reported as aquatic, is strictly terrestrial so far as the writer 

 has ever observed. In this respect, therefore, the fauna of this 

 loess is typical. 



However, it presents certain very interesting variations. The 

 difference between the fossils of the red and white loesses at 

 Helena, as shown in the appended table of fossils, is not very 

 great, but the red loess shows a smaller number of species, and 

 its fauna taken in the aggregate is rather less typically southern 

 than that of the white loess. When we compare the fauna of the 

 Helena loess with that of the Forrest City region, however, we 

 find a greater difference, and the latter is clearly northerly in its 

 affinities. It is evident that the valley of the L'Anguille river 

 formed a barrier which was not passed by several southerly 

 species. 



A comparison of the Helena loess fauna with that of Natchez, 

 Mississippi 7 , shows that they are very similar, only three of the 

 Natchez species (Polygyra inflecta, Vitrea hammonis, and Co- 

 chlicopa lubrica) being absent at Helena, though found at For- 

 rest City. Eight of the Helena species are not found at Natchez. 



The following list of fossils from Crowley 's Ridge adds sixteen 

 species to the lists published by Call. The figures indicate the 

 number of specimens collected. Those marked + were reported 

 by Call. 



7 See the following for the Natchez list: American Geologist, Vol. XXX, 

 1902, p. 290; and Bulletins from the Laboratories of Natural History, State 

 University of Iowa, Vol. V, 1904, p. 310. 



