Sterki: North American SpH.iiRiiD.E. 463 



95. Pisidium monas Sterki. 



Pisidium monas Sterki, Nautilus, XIV, 1900, p. 5. 



Possibly a variety of P. imbecille, but intermediate forms have not 

 been seen. 



Habitat. — Northern Michigan, Maine. 



96. Pisidium abyssorum (Stimpson) Sterki.^ 



Pisidium abyssorum (Stimpson) Sterki, Nautilus, XI, 1898, p. 124. 



Habitat. — Dredged from deep water in Lake ]\Iichigan and also 

 taken from the stomachs of whiteflsh. 



P. levissimum Sterki, Nautilus, XX, 1906, p. 18, somewhat re- 

 sembles P. abyssorum in size and shape, but may be distinct. It 

 occurs in Wisconsin. 



97. Pisidium tenuissimum Sterki. 



Pisidium tenuissimum. Sterki, Nautilus, XIV, 1901, p. 99. 



Quite common and rather variable. In the past it evidently was 

 very common, and there are some fossil forms differing considerably 

 from recent ones. 



Habitat. — Region of the Great Lakes; Ohio (occurring in the Ohio 

 River drainage area). 



Fossil. — Maine, Ohio, IVIichigan, Illinois. 



98. Pisidum pauperculum Sterki. 



Pisidium. pauperculum Sterki, Nautilus, X, 1896, p. 64. 



Very variable. 



Habitat. — New England and New York to Ohio, Ontario, and 

 Minnesota. 



Fossil. — Maine, New Jersey, Ohio, Michigan, Ontario (partly in- 

 cluding varieties). 



99. Pisidium pauperculum nylanderi Sterki. 



Pisidium pauperculum nylanderi Sterki, Nautilus, XI, 1898, p. 125. 

 Habitat. — Maine, Ontario, Michigan. 

 Fossil. — Maine. 



100. Pisidium pauperculum crystalense var. nov. 



Mussel higher, beaks more prominent; shell transparent to trans- 

 lucent, almost colorless to amber-colored. (The name is derived from 

 Crystal Lake, Michigan.) 



* With respect to the name see Sterki, I. c. 



