APPENDIX 



THE MOLLUSCA OF THE OKOBOJI REGION 



The varied physical and ecological conditions described in the 

 botanical part of this paper offer a variety of habitats to mol- 

 lusks and make this fauna of unusual interest. The abun- 

 dance of lakes and swamps has naturally led to the development 

 of a rich aquatic fauna which is marked, however, by great num- 

 bers of individuals rather than of species. These may be rough- 

 ly divided into lacustrine and palustral species, the former occu- 

 pying the more permanent bodies of water, such as lakes and 

 larger ponds, and the latter the borders of the lakes, the smaller 

 kettleholes, and the swamps and bogs. A third aquatic fauna 

 of the fluviatile type is furnished by the Little Sioux river on the 

 west, and the Des Moines river only a few miles east of our ter- 

 ritory, the latter being richer in species. The comparative study 

 of the fauna of these two streams is rendered especially inter- 

 esting because the streams belong to distinct drainage systems, 

 as has been noted. 



The freshwater Pulmonates are practically restricted to the 

 shallower waters or to the more prominent belt of submerged 

 aquatic vegetation, which does not exceed 30 feet in depth. The 

 gill-bearing forms, both univalve and bivalve, are found mostly 

 in more permanent bodies of water, and often extend to greater 

 depths. 



The dry prairie furnishes very few species and individuals, 

 and all are terrestrial Pulmonates. In the occasional clusters of 

 Symphoricarpos occidentalis a few specimens of Yallonia gracili- 

 cosia, Bifidaria holzingeri, Zonitoides minuscuhis, and more rare- 

 ly Yallonia parvida, Bifidaria armifera, etc., may be found, but 

 on the whole the true prairie is devoid of mollusks. The more 

 or less xerophytic forest borders show a larger number of both 



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