ORTMANN: monograph of the naiades of PENNSYLVANIA. 253 



Rhoads, 1S99 (Ohio River, Coraopolis, Allegheny Co., and Beavor, Beaver Co.). 

 Ortmann, 19096, pp. 191 and 202. 



Characters of the shell: Shell very large, moderately thick. Outline subovate, 

 but generally with a high posterior wing, which makes the outline subtriangular. 

 This wing is very variable, but rarely (in old shells) indistinct. Sometimes there 

 is an indication of a small anterior wing. Shell symphynote at the wings. Anterior 

 margin rounded. Lower margin slightly convex, or almost straight posteriorly. 

 Upper margin straight, ascending posteriorly, and forming with the posterior 

 margin, the elevated wing-like angle. Posterior margin obliquely descending, 

 slightly concave just below the angle, or straight, and passing in a broad curve 

 into the lower margin, so that there is no lower posterior angle to the shell. Beaks 

 low, hardly elevated above the hinge-line, located in the anterior section of the 

 shell. Beak-sculpture indistinct, feeble, consisting of three or four fine bars, the 

 first subconcentric, the following ones double-looped, with the anterior loop almost 

 effaced. Valves rather compressed, gently convex, flattened upon the sides; no 

 posterior ridge, but shell in this region broadly rounded, with one or two fine 

 radiating, elevated lines. Posterior slope compressed or slightly excavated, ele- 

 vated into the posterior wing. 



Epidermis thick, dark, in young specimens sometimes dark gi-eenish, but 

 generally dark brown to black, without any rays. Mere traces of rays are rarely 

 visible in young specimens. Growth-rests are generally not marked by darker color. 



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

 moderately thick, triangular, ragged. No interdentum. Laterals long, generally 

 curved, moderately strong. Beak-cavity shallow. Dorsal muscle-scars in an 

 irregular, almost vertical row in the beak-cavity. Adductor-scars distinct, the 

 anterior impressed, the posterior less so. 



Nacre always purple, generally very dark purple, rarely light purple, and in 

 one case I have seen a pale salmon-color, whitish toward the margins of the shell. 



Sexual differences present in the shell, but less marked than in Paraptera 

 fragilis. The female is slightly more swollen in the posterior part, of the shell, 

 and the lower margin is more broadly rounded in the postbasal region. These 

 differences, however, are slight, and not always reliable. Further, it seems that 

 the female does not attain the extreme size of the male. 



if no living shells are found, dead ones are very conspicuous. Its uppermost limit in the Allegheny is 

 Templeton in Armstrong Co., and above Pittsburgh it is in the Allegheny altogether a rare shell. 



