Occasional Papers of the Museum of Zoology 3 



ing species in the families and genera indicated : Dr. Bryant 

 Walker, Ancylidae, Amnicolidae and Unionidae; Dr. \ictor 

 Sterki, Sphseriidae, Valloniidae, Pupillidae; Dr. H. A. Pils- 

 bry, Succineidae ; Dr. F. C. Baker, the smaller Planorbes. 



All 1920 material, with the exception of Ferrissia. Amni- 

 cola and Planorbis (identified by Dr. Walker), have been 

 identified by comparison with that of the preceding year. 



NoT'Es ON Habitats 



A detailed description of the Devils Lake and Turtle Moun- 

 tains region has been published by the Geological Survey," but 

 a general outline drawn largely from that source may be of 

 interest here. Devils Lake lies in the northeastern part of the 

 state about a hundred miles west of Grand Forks. It is in 

 the Drift Prairie Plains, between the Red River \'alley and 

 the Great Plains Plateau. It is a brackish lake of glacial ori- 

 gin, extremely irregular in outline and shallow; and in 1919 

 it was about fifteen miles in greatest length and four and a 

 half miles at its greatest width. From year to year the water 

 level varies, having receded in all 14.13 feet during, the period 

 between 1883, when it was first surveyed, and 1912. Fluctua- 

 tion of level is characteristic of all the small lakes of the re- 

 gion, though some of them remain fresh enough, by reason of 

 an adequate water-shed, to support a molluscan fauna. No 

 snails or clams inhabit Devils Lake or waters of similar alka- 

 line composition, but bleached snail shells on old beach lines 

 indicate once flourishing colonies. 



The Turtle Mountains, with an area of 600 to 800 square 

 miles, extending into Canada, form a group of low morainic 

 hills interspersed with glacial lakes. The distance to be cov- 



^ Simpson. Howard E., The Physiography of the Devils-Stump 

 Lake Region, North Dakota. Sixth Biennial Report, A'". D. State Gcol. 

 Siirv., igi2, pp. 103-157. 



