THE NAUTILUS. 81 



Lake Michigan : Charlevoix ("Walker) and the South end (Daniels) 

 is somewhat intermediate, although more like the Idaho form, and 

 so it appears to be safer to regard the above as a variety of the same 

 species for the present. 



Pisidium compression J'r., is very variable, but a number of its 

 forms and varieties are characteristic and rather constant. As 

 typical is accepted the common river and creek form : beaks high, 

 narrow, with well developed appendages, above which there are 

 small flattened or even impressed smoothish areas, usually with more 

 or less distinct radial lines ; balance of the surface with rather 

 coarse, sharp, regular, concentric striae, dull, with microscopic 

 wrinkles, color whitish to grayish, and often there are marginal 

 zones of straw to yellow color, with more shallow, irregular striae, 

 more or less shining; shell and hinge stout, with whitish nacre. 



Var. opacum n. In shape and size near, the typical form, well 

 inflated, but the surface is finely and irregularly striate, dull to shin- 

 ing, color often plumbeous above ; beaks with the appendages 

 slighter, or merely flattened on top ; shell and hinge stout, the for- 

 mer opaque. 



This is a form of sloughs, ditches, pools, etc., along rivers and 

 creeks, quiet places in such, with muddy bottom, also of lakes and 

 ponds near inlets; it seems to be a retrograde one, with respect to the 

 surface sculpture, and it is notable that also the young in such places 

 have the fine, obsolete striae. 



Var. Icevigatum n. Moderately oblique, of medium to rather 

 large size, generally well inflated ; beaks less elevated, rounded or 

 more or less flattened on top with slight or obsolete ridges ; surface 

 with fine, irregular striae to nearly smooth, more or less shining ; 

 color light to dark horn; shell slight, translucent, nacre, more glassy; 

 hinge slight, generally less angular than in the type. 



Widely distributed, in quiet waters. These mussels often have 

 considerable resemblance, in shape, with Pis. variabile Pr. (which is 

 variable on somewhat corresponding lines), and it is sometimes very 

 difficult to distinguish dead or fossil specimens of the two, and even 

 fresh shells of certain forms. 



Var. limnicolum n. Near fcerigatum, but much smaller, sonic 

 specimens have ridges or well-formed appendages on the beaks; 

 shell and hinge slight, cardinal teeth well formed. A form of 

 deeper, quiet water, lakes, etc. Hundreds of specimens at all stages 



