THE NAUTILUS. 79 



MOLLUSCA OF SOUTH DAKOTA. 



BY W. H. OVER. 



Inasmuch as practically nothing has been published regarding 

 the Mollusca of this State, I deem it proper to submit a list, 

 prefaced with a few remarks in explanation of the environic 

 conditions for shell-life as they exist today, and to some extent 

 in the past. 



During the last decade I have collected quite generally over 

 the State, yet I realize there is much study and work to be done 

 and in time the following list will be greatly increased. How- 

 ever from this locality, one would hardly expect a long list of 

 species. 



The surface of South Dakota is usually spoken of as a rolling 

 plain, destitute of timber, except in the Black Hills and the 

 Forest Reserves in the northwest part of the State. There is, 

 however, considerable timber along streams locally over the State, 

 and the eastern part is dotted over with large groves and is really 

 in the humid district ; however, strictly speaking, only the south- 

 eastern corner is considered so. Nearly one-half of the State that 

 lies west of the Missouri River, except the Black Hills, is con- 

 sidered semi-arid, and owing to the nature of the soil the surface 

 has been eroded into deep-cut creeks and steppes, and merges 

 into a large area of " badlands ". 



The average rainfall is about 14 inches in the northwestern 

 part of the State and increases to 30 inches in the southeastern 

 part. 



The altitude in the southeastern part is about 1240 feet above 

 sea level and increases toward the west, where at Harney Peak 

 in the Black Hills, it is 7200 feet. However the lowest is 960 

 feet in the northeastern part around Bigstone Lake where a 

 small area was probably scooped out by the glacier, and drains 

 into the Minnesota River Valley. The balance of the State is 

 drained by the Missouri River which nearly divides the State 

 east and west. The eastern half is drained from north to south 

 by the Big Sioux, Vermillion and James rivers. The western 

 half is drained from west to east by the White, Cheyenne, 



