ORTMANN: monograph of the naiades of PENNSYLVANIA. 321 



Characters of the shell: Of medium size to rather large (but not as large as 

 L. ovata), extremely thick and heavy (-one of the heaviest shells for its size). Out- 

 line subelliptical or subovate, very variable and often irregular, chiefly so in the 

 female, always less than one-and-a-half times as long as high. Anterior margin 

 rounded. Lower margin convex, sometimes rather uniformly so, but in other 

 cases the middle part of the lower margin is less convex, and the posterior part 

 slopes up rather steeply. Upper margin short, gently convex, passing gradually 

 into the obliquely descending posterior margin, without forming a distinct angle. 

 Posterior and lower margins uniting posteriorly in a blunt angle, which may be 

 obliterated. Beaks moderately swollen, and little projecting above the hinge- 

 line, more or less inclined forwards, and located in front of the middle of the shell. 

 Beak-sculpture obsolete. Although I possess specimens with fairly well preserved 

 beaks, I cannot see any distinct beak-sculpture. Valves more or less convex, 

 generally moderately so, but old specimens, chiefly females, are much more convex 

 (almost globular). The convexity is least upon the sides, greatest in the region 

 of the posterior ridge. The latter, however, is quite indistinct. Posterior slope 

 slightly convex or flattened, narrow. 



Epidermis brown, light or dark yellowish browoi or reddish brown, generally 

 without any trace of gi-een shades. This color is rather uniform, and no concentric 

 bands of light and dark are present. Rays either absent (chiefly in older shells), 

 or, when present, faint and indistinct. In young, well-preserved specimens, the 

 rays are fine or moderately wide, but not dark (black or dark green), but gray or 

 grayish green, or brownish. They are not sharply contrasted with the ground- 

 color, and disappear toward the lower margin, so that older shells have rays only 

 near the beaks. In very old specimens the general color becomes uniformly dirty 

 brown, but not very dark brown. 



Hinge well developed. Pseudocardinals two in left, one or two in right valve 

 (the anterior one smaller), variable in shape, strong, stumpy and ragged. Inter- 

 dentimi narrow. Laterals moderately long, slightly curved, thick and heavy. 

 Beak-cavity moderate. Dorsal muscle-scars in the beak-cavity. Adductor-scars 

 distinct, all deeply impressed, the posterior scars less so. Nacre silvery white, 

 very often with more or less pink, chiefly in the beak-cavity and near the hinge, 

 and sometimes the whole interior of the shell is suffused with beautiful salmon or 

 pink. But in other cases the pink shades may be entirely absent. 



Sexual differences of the shell very strongly marked. In the male the out- 

 line is rather regularly subovate or subelliptical, with the lower margm rather 

 regularly curved, meeting the posterior margin in a blunt point. In older males 



