ORTMANN: monograph of the naiades of PENNSYLVANIA. 233 



Marshall, 1895 (Allegheny River, Warren Co.). 



Rhoads, 1899 (Ohio River, Coraopolis, Allegheny Co., and Beaver, Beaver Co.; Beaver River, Wampum, 



LawTence Co.). 

 Ortmann, 19096, p. 190. 



Characters of the shell: Shell large, thick, and heaw. Outline subclliptical or 

 subovate, more or less elongated. Anterior margin rounded, lower margin and 

 upper margin together with the posterior margin forming rather regular curves, 

 joining posteriorly in a blunt point, situated below the horizontal middle line of 

 the shell. Beaks moderatelj^ swollen, little elevated, situated anterior to the 

 middle of the shell. Beak-sculpture poorly developed, consisting of a few fine, 

 indistinct bars, which have a tendency to be double-looped. Valves moderately 

 and rather uniformly convex; posterior ridge faint, indistinct, or obliterated. No 

 sculpture upon the disk. 



Epidermis yellowish, greenish, or brownish-olive, often with more or less 

 distinct rays. Rays present chiefly in young individuals, broad and continuous, 

 sometimes sharply marked, sometimes obscure, covering the whole shell. In other 

 cases, there are hardly any traces of rays, and in old specimens the epidermis 

 is uniformly brown or blackish. Concentric bands of color are sometimes present. 



Hinge well-developed. Pseudocardinals normally one in right, two in left 

 valve, but often additional ones are present. The normal pseudocardinals are 

 strong and heavy, ragged, subtriangular, divergent. Interdentum narrow, rather 

 long. Laterals long, strong, heav>\ Beak-cavity moderate. Dorsal muscle-scars 

 in beak-cavity and upon the hinge-plate. Adductor-scars distinct, well impressed, 

 chiefly the one anterior. Nacre silverj' white, but often discolored, in very rare 

 cases pinkish.'" 



Sexual differences present, but slight. In the male the lower margin is rather 

 evenly curved. In the female it is more curved out in its posterior part, so that 

 the poterior point of the shell is more elevated, and the whole posterior end of the 

 shell appears higher and more rounded. But in many cases, it is hard, or even 

 impossible, to tell the sex from the shape of the shell. Wherever the posterior 

 expansion is distinctlj^ developed, we may be sure to have a female before us. 



L. H. D. 



Size: 1. Edinburg, Cat. No. 61.3491 (9 gra\4d) 155 mm. 95 mm. 61 mm. 



2. Pulaski, Cat. No. 61.4059 ( 9 ) 147 " 94 " 63 " 



3. Edinburg, Cat. No. 61.3491 (cf ) 124 " 76 " 51 " 



4. Cooks Ferry, Cat. No. 61.4791 (cf ) 93 " 65 " 41 " 



'■" Such specimens have never been found in Pennsylvania, but I have a specimen with pinkish 

 ■ nacre from Blennerhasset Island, near Parkersburg, West Virginia. 



