248 MEMOIRS OF THE CARNEGIE MUSEUM. 



posterior loop distinct, while the anterior is obliterated. Valves rather compressed, 

 moderately and rather uniformly convex, slightly more flattened upon the sides. 

 No distinct posterior ridge. Posterior slope compressed, and even somewhat 

 excavated, chiefly when the upper posterior wing is well developed. 



Epidermis smooth, light-colored, pale yellowish to pale greenish, sometimes 

 light olive-brown, with or without greenish rays. When present, the rays are 

 straight, continuous, greenish, rather narrow, and not well-defined. On the 

 posterior slope, the epidermis is lameUar, darker, often with indications of a few 

 stronger rays. Growth-rests generally distinctly indicated by dark concentric 

 bands. 



Hinge poorly developed. Pseudocardinals two in left, one in right valve, 

 feeble, subtriangular and lamellar, compressed, often imperfect and represented 

 by mere ridges. Interdentum absent. Laterals long and thin, the lower one in 

 left valve often rudimentary. Beak-cavity shallow. Dorsal muscle-scars in an 

 oblique row in the beak cavity, but rather far from its extreme point. Adductor- 

 scars faintly impressed, the anterior ones more distinct than the posterior ones. 

 Nacre silvery white, in most cases with more or less pink, chiefly towards the 

 beak-cavity; sometimes entirely pinkish. 



Sexual differences distinctly marked in the shell. In the male the lower 

 margin is regularly convex, curving up in its posterior half, and the posterior end 

 of the shell is rather narrowly rounded. In the female the lower margin is more 

 expanded in the postbasal region, so that a greater anterior section is descending, 

 while posteriorly it curves up in a very broad curve, rendering the posterior end of 

 the shell more broadly rounded. In the male the greatest height of the shell is 

 located more in the middle of the shell, in the female, it lies in its posterior portion. 

 The posterior expansion of the female is generally extremely thin. 



L. H. D. 



Size: (Males) 1. Erie, Cat. No. 61.4807 116 mm. 78 mm. 38 mm. 



2. Industry, Cat. No. 61.3549 100 



3. Erie, Cat. No. 61.4808 99 



4. Industry, Cat. No. 3549 67 



(Females) 5. Erie, Cat. No. 61.4109 100 



6. do. Cat. No. 61.4807 (gravid) 84 



7. Edgeworth, Cat. No. 61.1360 82 



8. Industry, Cat. No. 61.3549 (gravid) .... 67 



The largest specimens at hand both from Lake Erie and the Ohio system are 

 given. This species does not reach its maximum size in Pennsylvania, and, as is 

 shown above, the specimens in Lake Erie are larger than those from the Ohio. 



