68 THE NAUTILUS. 



well inflated; beaks not much posterior, rounded or slightly flattened 

 on top, well prominent over the hinge margin ; the latter slightly 

 curved in the adult, almost straight in the young and half-grown, 

 with projecting, not or hardly rounded angles at the scutum and 

 scutellum, which are slightly to well marked, narrow; posterior mar- 

 gin subtruncate above, passing into the well rounded inferior with 

 an uninterrupted curve, or with a slightly marked, rounded angle, 

 more so in the young; supero-anterior margin little to moderately 

 curved, sloping from the projecting angle at the scutellum to the 

 rounded anterior end ; surface regularly and rather coarsely striated, 

 dull, rarely somewhat shining in older specimens ; epiconch thin and 

 often worn off, pale horn-colored in the young, lighter to darker 

 grayish to brownish in older specimens, usually with a lighter zone 

 along the margins; shell moderately thick, nacre glassy, colorless to 

 white or bluish, muscle insertions distinct; hinge stout, plate rather 

 broad, cardinal teeth well formed, short, the right one rather strongly 

 curved, its posterior end thickened and grooved, the left anterior 

 angular, stout, the posterior small, oblique ; lateral teeth rather short, 

 stout, their cusps short, pointed, the outer ones in the right valve 

 well formed ; ligament short, strong. 



Size: long. 5, alt. 4.4, diam. 3.4 mill. 



Habitat: New York to Ohio, Michigan, Illinois and Minnesota, 

 rather common in creeks, rivers and small lakes. 



Well formed specimens are easily recognizad by the oblique shape, 

 the rather short, slightly curved hinge margin with the projecting 

 angles at both ends, the regular striation and the dull, often roughish 

 appearance of the surface ; by the latter features it may be discerned 

 from some forms of P. scutellatum which are of similar shape ; but it 

 is more nearly related to P. nov-eboracense and affine ; and some full- 

 grown, well-inflated specimens, in which the scutar and scutellar 

 angles are sometimes less marked, might be mistaken for one or the 

 other. But a lot of mussels at different stages of growth are always 

 recognizable at once ; the projecting angles, especially marked in 

 half-grown specimens, in connection with the peculiarly dull surface 

 (like in "typical" P. compressum and in P. kirklandi) are well 

 marked characters, and so is the color, which turns to grayish while 

 the mussels are much younger and smaller than in P. affine. In the 

 latter species, the hinge-teeth, especially the cardinals, are longer 

 and finer. 



