ORTMANN: monograph of the naiades of PENNSYLVANIA. 137 



On May 23, 1912, I found a single specimen of this species in the headwaters 

 of the Monongahela in West Fork River, Lightburn, Lewis Co., West Virginia. 

 I found it buried deeply in sand and gravel held together by the rhizomes and roots 

 of Dianthera. 



Being thus present in two rivers, which run into our state, it might have once 

 existed here, or else it accidentally may have escaped detection. 



The systematic position of this genus is as yet quite uncertain. The account 

 of the anatomical structure given by Simpson is quite unsatisfactory, in fact 

 partly unintelligible. Besides the nomenclature of the genus requires final adjust- 

 ment. 



Genus Anodonta Lamarck (1799). 



Ortmann, 1912, p. 286; Simpson, 1914, p. 358. 



Type Mytilus cygneus LinnjEus. 



A large number of species of this genus are known, many of which may be 

 only special phases of others. In Pennsylvania, four species and one or two 

 varieties may be distinguished, but I am not sure whether all of them are "good 

 species." 



Key to the Species and Varieties of Anodonta in Pennsylvania. 

 a I. Umbonal region convex, or more or less inflated and elevated above the hinge-line. Beak-sculpture 

 consisting of several rather strong, double-looped bars, which generally have a sharp re-entering 

 or angular sinus. Gonochoristic. 

 bu Beak-sculpture rather heavy, forming distinct nodules on the posterior loop. 

 Ci. Shell rather compressed, not swollen. 



di. Shell not very thin, ovate, somewhat elongate A. grandis. 



d-2. Shell rather thin, large, higher and shorter ,-1. grandis gigantea.^^ 



Co. Shell more swollen and inflated, chiefly in the umbonal region A. grandis footiana. 



62- Beak-sculpture less heavy, generally not nodulous, and bars of uniform thickness. 



Ci. Shell rather thin or moderately thick, not thickened along the lower anterior margin. 



A. cataracia. 



Co. Shell somewhat thicker, and especially so along the lower anterior margin .4. implicata. 



«o. Umbonal region remarkably depressed and flattened, and not elevated above the hinge-line. Beak- 

 sculpture consisting of several delicate, irregular bars, with a slight tendency to fall into two loops, 

 the sinus being a mere notch. Hermaphroditic .4. ohie7isis. 



*' I shall not treat this as a " variety." It has been introduced here only to bring out its characters. 



