THE NAUTILUS. 83 



part (plate) rather long and slightly curved ; cardinal teeth small 

 and rather defective, 1 of the right, the posterior end is small, simple, 

 the anterior small, low and nearly obsolete, the left anterior short, 

 somewhat massive, the posterior oblique, short, slight and not pro- 

 jecting over the level of the valve-edge, lateral teeth at strong angles 

 to the plate, the right ones little elevated, the cusps of the left 

 rounded ; ligament stout, covered in younger, partially or quite un- 

 covered in adult specimens. 



Size : long. 4, alt. 3.8, diam. 2.8 mill, (average). 



Habitat: Blue Lake, Muskegon co., Michigan, collected by Dr. 

 Kirkland, in good numbers, remarkably uniform. 



This is certainly a remarkable form, and has rather the significance 

 of a species, yet seems to be connected with other forms of P. com- 

 pressum. 



Var. coosaense n. Of medium size, straw to light horn colored ; 

 in shape near the typical form, with rather posterior, narrow, ele- 

 vated beaks bearing appendages ; superior margin somewhat less 

 curved and rather long, the supero-anterior slope somewhat more re- 

 mote from the beaks ; surface with fine to very fine striae, dull to 

 slightly shining. Shell and hinge rather strong. 



Hab.: Georgia and Alabama, along the Coosa river (collected by 

 Smith, sent by B. Walker). 



In younger to two-thirds grown specimens, the features of this 

 form are especially well marked : the outlines rather angular, all mar- 

 gins little curved, while full-grown examples are approaching the 

 typical form, except as to surface sculpture. 



Var. contrarium n. Of medium size, well inflated, moderately 

 oblique ; outlines various from near those of coosaense and the typ- 

 ical form, though less angular, to rounded, nearly oblong or oval, 

 with only the supero-anterior slope more or less marked ; beaks gen- 

 erally less posterior, moderately elevated, rather narrow, with a 

 small, more or less flattened area on top, and a slight ridge or even 

 small appendage ; surface with rather fine, crowded, sharp, regular 

 stria?, dull ; color pale to grayish horn, often with a light zone along 

 the margins ; shell and hinge rather stout. This Pisidium, although 

 .rather variable in itself, and probably connected with other forms, 

 marks an interesting contrast to coosaense as well as to the type, and 

 laevigatum, etc. 



Alabama : Attalla, Calera, Ebenezer Church, Montevallo. (Smith 

 collected, B. Walker sent.) 



Var. smithii n. Of rather good size, little inequipartite, somewhat 

 oblique, moderately to rather well inflated, more so near the beaks, 

 while the lower parts of the disks are rather flat ; outlines rather 

 angular, with the angles more or less rounded, superior and inferior 

 margins moderately curved, the superior rather long ; supero-anterior 

 slope nearly straight, steep, posterior part subtruncate ; beaks not 



1 In several specimens examined. 



