OF THE MOLLUSCA OF IOWA. 63 



great abundance in the loss of certain species now rare or 

 extinct in those localities is very significant as illustrative 

 of continuous change in thegeographic distribution of spe- 

 cies. The relation of these fossils to the living represen- 

 tatives in the same area is also of great interest as being 

 indicative of the climatic conditions at the time of the 

 deposition of the loss. As has been shown by McGee and 

 CalP the forms occurring in this deposit are all depauper- 

 ate, attesting a diminution of vitality, doubtless due to a 

 much lower temperature than at present. It appears from 

 the data at hand that, in Iowa, among the laraellibranchs 

 especially, and to a less noticeable degree in the gastero- 

 podous groups, thedistribution of the speciesgeographically 

 is very peculiar. Among the Uiiionidce alone, thirty-five 

 species apparently occur only in the eastern part of the 

 state, eight only in the western, while twenty-four are gen- 

 erally distributed : of the latter, one is extremely rare in 

 western, and three in eastern Iowa. Independent of its 

 zoological interest this peculiar distribution of the mollusca 

 of Iowa is geologically very suggestive as affording evi- 

 dence of certain phenomena during glacial times. Iowa 

 forms a portion of an area, the molluscan fauna of which 

 has been derived preglacially from the northwestward. The 

 recent rich discoveries of fresh-water mollusca in the Mes- 

 ozoic and later deposits of the Upper Missouri region have 

 indicated the probable origin of the fauna now living within 

 the limits of this and contiguous states. In the fresh-wa- 

 ter cretaceous and tertiary strata just referred to, are found 

 the prototypes of the legamentinus, gihbosus, undulaius, 

 ventricosus, ellipsis and other groups of the Unionidce; 

 while Planorbis, Campeloma, and other gasteropodous 

 genera were also well represented by forms closely allied 



» Am. Jour. Sci., Sept., 1882, Vol. xxiv. 



