94 Natural History .Bulletin. 



high-altitude species, as well as in that of depauperate LimncBa 

 ■caperata (resembling exactly the recent shells from Nebraska 

 prairie ponds which often dry up), which are smaller and more 

 slender than specimens from colder, moister regions far to the 

 north. 



The size and development of all of these forms must have 

 been governed, as it is now, by the abundance of food and the 

 length of the period of activity. The amount of food, as well 

 as the length of the warm, moist period of activity would be 

 less in a dry, as well as in a cold climate, and it is impossible 

 to state definitel}' whether only one or both of these conditions 

 were responsible for the observed variation, though the 

 modern habits of some of the species indicate that we should 

 not entirely ignore the former one of these conditions. On 

 the other hand the snr.alier, heavier forms of Mesodon nmlti- 

 lineata and Succinea obliqua^ Vertigo ovata, the Limncece^ 

 etc., are associated with an abundance or excess of moisture, 

 and rather point to the presence of even more moisture than 

 now occurs in the region under consideration. The great 

 majority of the molluscs, however, indicate a climate at least 

 as moist as that of this region, while the presence of boreal 

 species points to a temperature somewhat below the present in 

 average, — an opinion which is only strengthened by the wide 

 distribution (especially in a northerly direction), and the great 

 adaptability to climatic variations of the majority of the 

 remaining species. 



The climate was comparatively uniform during the entire 

 period, if we may judge from the fossils, for the same forms 

 are often found from the very base, just over the drift, to the 

 uppermost portions of deposits one hundred feet or more in 

 thickness. No difference is noticeable between specimens of 

 the same species from the lowermost and the uppermost por- 

 tions of the deposits, — a fact which warrants us in concluding 

 that climatic changes during the deposition of the Loess could 

 not have been very great. The fact may here again be em- 

 phasized that while a depauperation is noticeable in some of 



