Sterki: North American Sph.eriid.e. 447 



5. Pisidium compressum Prime. 



Pisidium compressum Prime, Boston Proc, IV, 1851, p. 161 ; Mon. Corb., 1865, p. 64. 



Habitat. — ^Distributed over the continent, commoner in the East 

 than in the West. The "river and creek form" is regarded as typical. 



Fossil. — Maine, New Jersey, Ohio, Michigan, Illinois, South Dakota, 

 New Mexico; mostly the var. Icevigatum and intermediate forms. 



6. Pisidium compressum opacum Sterki. 



Pisidium compressum var. opacutn Sterki, Nautilus, XIX, 1905, p. 81. 

 Habitat. — New Jersey, Ohio, Illinois. 



7. Pisidium compressum curvatum var. nov. 



More oblique, less high, less inflated, the posterior and inferior 

 margins forming one continuous curve; striae rather coarse and sharp 

 to fine and nearly obsolete; color less whitish, more corneous or tan; 

 the "New England form." 



Habitat. — Maine, Massachusetts. 



8. Pisidium compressum pellucidum var. nov. 



Smaller, less elevated, beaks less pointed, with the ridges slight or 

 vestigial ; surface with the striae slighter, more or less glossy, shell 

 translucent. 



Habitat. — Ontario, northern New York, Michigan. 



9. Pisidium compressum arrosum Sterki. 



Pisidium compressum var. arrosum Sterki, Nautilus, XIX, 1905, p. 82. 

 A small, slight, lacustrine and deep-water form. 

 Habitat. — Michigan, Wisconsin. 



10. Pisidium compressum rostratum Sterki. 



Pisidium compressum var. rostratum Sterki, Nautilus, XIX, 1905, p. 82. A lake 

 form. 

 Habitat. — IVIichigan, Indiana. 



1 1 . Pisidium compressum conf ertum Sterki. 



Pisidium compressum var. confcrtum Sterki, Nautilus, XIX, 1905, p. 82. 

 Habitat. — Blue Lake, Muskegon County, Michigan. 



12. Pisidium compressum Isevigatum Sterki. 



Pisidium compressum var. Icevigatum Sterki, Nautilus, XIX, 1905, p. 81. 



Widely distributed; the form of quiet waters. Probably ancestral. 

 Fossil. — Found in many places. See under P. compressum. 



