The Lcess and its Fossils. 



93 



of the Loess surface as corresponding to our modern prairies, 

 since the same species now sparingly occur in the narrow 

 bands of timber along our prairie streams. In fact their pres- 

 ence only makes the similarity of the Lcess molluscan fauna 

 to that of the modern prairies, taken as a whole, much more 

 striking. 



II. Discussion and Conclusions. 



The presence of the boreal species', the prevalence of other 

 rather northerly forms, the presence of the larger forms of 

 Zonitesfulvus and l^a/lonia pu/c^eUa {corrtsponding to larger 

 specimens from high altitudes in Montana and Nevada), the 

 abundance of Helicina occulta^ which, though not boreal, now 

 prefers cool, northern hillsides (to which it has probably 

 retreated as a last resort in the limited localities in which it 

 now occurs), and the stunted form of the species specially 

 referred to, all indicate that the climate was then colder than 

 at present^ though its severity could by no means have been 

 such that ice-bound lakes and streams could exist during the 

 summer. The mingling of northern and middle latitude 

 species would rather indicate a mean between the climate of 

 the northern and that of the Interior Regions. 



It is interesting to consider in this connection the extent of 

 variation and degeneration which would be induced by a dry, 

 as well as by a cold climate. 



Certainly those who maintain that Baron Richthofen's the- 

 ory (which presumes a dr\- climate) is applicable to the Loess 

 under consideration would tind some support for their views 

 in the occurrence of Succinea lineata^ Patida strigosa cooperi, 

 and Pupa alticola^ which now seem to be dry- climate and 



1 As already noted, Pupa muscoritm, while a boreal species on this conti- 

 nent, is not so in the old world. This reduces the number of strictly boreasl 

 species. 



2 This has already been suggested by McGee and Call in the American 

 Journal oj SdeHce,Yo\. XXIV., September, iSSj. A comparison of large sets 

 of both fossil and recent shells does not however admit of the degree of differ- 

 ence in climate there suggested. 



