ORTMANN: monograph of the naiades of PENNSYLVANIA. 



277 



margin, which joins the lower margin in a distinct, but rounded, angle, so that the 

 shell is more or less narrowed and pointed at the posterior end. Beaks low, only 

 little elevated above the hinge-line. Beak-sculpture faint and obsolete, con- 

 sisting of three to five indistinct bars, which are double-looped, with a sinus in 

 the middle; the posterior loop is open, i.e., the bars disappear upon the posterior 

 slope. Valves moderately convex, flattened upon the sides. Posterior ridge 

 practically absent, only toward the beaks is it faintly indicated. Posterior slope 

 gently convex, flat towards the beaks. 



Epidermis glossy, dark green to brown or blackish, with indistinct rays or 

 without rays. Young shells are lighter green, with dark green rays of various 

 width, but in the posterior section of the shell the rays are generally rather wide. 

 In old sheUs, the rays become obscure, and the epidermis is often uniformly dark 

 brown or black. Sometimes concentric bands of dark color, marking growth- 

 rests, are present, but generally the growth-rests are not marked by color. 



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

 stumpy or slightly compressed, crenulated. No interdentum. Laterals long, 

 strong, rather straight. Beak-cavity shallow. Dorsal muscle-scars in beak-cavity. 

 Adductor-scars distinct, those anterior well impressed, those posterior less so. 

 Nacre white, iridescent posteriorly. Often there is more or less pink or light 

 purple in the beak-cavity and upon the hinge teeth; sometimes aU of the nacre is 

 purple, but such specimens have not been found in Pennsylvania. 



Sexual differences of shell well marked. In the male, the shell tapers pos- 

 teriorly rather uniformly to a point, and the posterior part is distinctly narrower 

 than the anterior. In the female, the lower margin is distinctly enlarged in the 

 postbasal part. In front of this widening, the lower margin is straight or shghtly 

 concave, behind it slopes up rather suddenly to the posterior point, which, in 

 consequence of this, is more elevated above the base line than in the male. The 

 posterior section of the female shell is not distinctly narrowed, and sometimes even 

 slightly higher than the anterior. However, in some females, the characteristic 

 shape is not very weU developed. 



L. H. D. 



Size: (Males) 1. Aladdin, Cat. No. 61.3522 177 mm. 70 mm. 49 mm. 



2. Neville Island, Cat. No. 61.706. 



3. Industry, Cat. No. 61.4077 



4. Mosgrove, Cat. No. 61.4075. . 

 (Females) 5. Cooks Ferry, Cat. No. 61.3519, 



175 



100 



71 



.. ..156 



6. Industry, Cat. No. 61.4077 140 



7. Industry, Cat. No. 61.3518 Ill 



8. Meadville, Cat. No. 61.3512 89 



(Lake Erie-form) 9. Erie, Cat. No. 61.4070 (cf) US 



10. do. " " do. (9) 112 



