THE NAUTILUS. 81 



THE MOLLUSCA OF DOUGLAS COUNTY, KANSAS. GASTEROPODA. 



BY G. DALLAS HANNA. 



As one travels westward from the Mississippi River, the gradual 

 diminution of the forest growths is very apparent. Tracts oi tim- 

 ber get smaller and smaller and the trees become more stunted in 

 growth until when the middle of Kansas is reached there are only a 

 few cottonwoods, confined to a narrow belt along the banks of 

 streams. Eastern Kansas is then a transition zone, where the tim- 

 bered regions of the east fade gradually into the vast semi-arid 

 plains of the southwest. 



A botanical condition such as this is bound to have a very great 

 influence upon a group of animals as closely associated with vegeta- 

 ble growths as the land mollusks are. We find some species — as 

 the majority of the Polygyras for instance — extending only as far 

 west as the trees go. Moisture and shade are requisites to their 

 existence, and we find them becoming rarer in direct proportion to 

 the decrease in vegetation. They have migrated westward just as 

 fast but no faster than the trees. 



And on the other hand there are some species of the plains area 

 which do not extend appreciably into the timbered regions. On the 

 border-line then, as we would expect, we find an intermingling of 

 the two faunas. 



Douglas County is a typical border county. Trees exist in little 

 patches here and there but are mostly confined to the near vicinity 

 of streams. Mollusks are not plentiful and generally distributed, 

 but are addicted to living in colonies in the patches of trees. 



Among the most important tracts of timber conchologically is 

 Blue Mound, situated six miles southeast of Lawrence. This little 

 hill wliich rises two hundred and fifty feet above the river flood 

 plain, supports an excellent growth of small trees. Under the dead 

 leaves and loose stones the small snails are to be found more abund- 

 ant than any place else in the county. 



The outcropping limestone ledges furnish several species which 

 belong in general to the southwestern plains fauna. Bulhnulus 

 dealbatus, Vallonia parvida, Bijidaria procera and Zonitoides 

 singleyana, are abundant in some such localities where there are no 

 trees for miles. 



Localities favorable tor tresh-water snails are rare. The only 



