g6 Natural History Bulletin. 



tically for many feet, as though the shells had gradually accu- 

 mulated through many generations. Again other species are 

 more generally and more uniformly distributed in a manner 

 which recalls their present habits, and which indicates that 

 they have not drifted into the places in which we find them to- 

 day, — at least not sufficiently far to disturb the arrangement 

 with reference to each other, which we may observe in the 

 living specimens. 



Drifted shells are usually either thrown upon the banks of 

 streams in comparatively large numbers, where they then 

 occur in narrow bands, or, if too heavy to float, they are 

 dropped long before such fine sediment as that which com- 

 poses the beds in which these fossils occur. Neither the hori- 

 zontal nor the vertical distribution indicate that these shells 

 had been removed to any considerable distance from their 

 original locations. The view which has been advanced by 

 the majorit}' of those who have worked upon the.Lcess of the 

 Mississippi and the Missouri (/. c. Owen', White', Todd% 

 Aughey', McGee', Witter', etc.) is that it was deposited in 

 large lakes or lake-like expansions of rivers. While the 

 homogeneity and the occasional lamination of the Loess indi- 

 cate a sub-aqueous origin, the character and distribution of 

 the fossils combat the idea of large bodies of water existing 

 during the summer — the growing-period of the molluscs. 



The writer's conception of the climate and of the origin of 

 the Loess, based largely upon a careful and extended study of 

 its fossils, may be briefl}^ summarized as follows: 



I. The summers during the formation of this deposit were 



1 On the authority of the following. 



2 I?eport of the Geol. Siir. of the State of lozva, by C. A. White, M. D. Vol. 

 I., 1870. 



3 Prof. J. E. Todd in the Proc. of the A. A. A. S., Vol.. XXVIl., 1S78. 



4 Sketches of the Phys. Geog. and Geol. of Nebraska, by Prof. S. Aughey, 

 1880. 



5 Am.your.ofSc{e7ice,Yo\.^^l\., Sept., 1SS2. W. J. McGee and R. 

 Ellsworth Call. 



6 Notes on the Loess, by F. M. Witter. 



