Dr. Hildreth on fresh water Shells. 287 



Length, 2.00 ; breadth, 3.5 ; diameter, 1.4. — Hab. Duck 

 Creek. 



Shell, thin and fragile ; anterior side, broad •, disks, con- 

 vex ; beaks, slightly elevated, and approximate ; ligament, el- 

 evated ; hinge margin, elevated, compressed, earinate ; basal 

 margin, a little shortened ; in young shells, rounded ; ante- 

 rior margin, narrow ; posterior, broad and rounded ; anterior 

 dorsal, subtruncate ; epidermis, greenish yellow, rayed with 

 dark green, and finely striated transversely ; surface, smooth 

 and glossy ; cardinal teeth, crenated and long ; cavity of the 

 beaks, small ; posterior muscular impression, broad ; naker, 

 bluish white, or pearl color. 



Remarks. — This is a very neat, and handsome shell — outer 

 surface remarkably clean, and free from parasitic plants* — 

 It is said to be very superior for eating. 



No. 21. Unio Ovatus. — Fig. 21. 



Shell, roundish ovate, convex, umboes elevated, beaks re- 

 curved, and approximate ; anterior lunule, flattened ; teeth, 

 crest-like, elevated. 



Length, 3.75 ; breadth, 5.00 ; diameter, 2.25 — Hab. 

 Muskingum. 



Shell, broader before, and narrower behind the beaks ; thin 

 and translucent when young ; and not thick when old ; disks, 

 swelled ; umboes, prominent ; ligament, partly concealed ; 

 anterior lunule flattened, and fuscous, becoming lamelar with 

 striae and wrinkles ; epidermis, yellowish, or horn color ; sur- 

 face, glabrous and shining, deeply wrinkled, and rayed in 

 young subjects ; cardinal teeth, crest like, elevated, compres- 

 sed, oblike, nearly on a line with the anterior dorsal margin ; 

 lateral teeth, short and elevated ; cicatrices, smooth and po- 

 lished ; cavity, large and somewhat angular ; naker, pearly 

 white. 



Remarks. — This is one of the most common shells in the 

 Muskingum, and remarkably uniform it its appearance. I 

 think it a near relation of the gracilis. 



No. 22. Unio Triangularis. — Fig. 22. 

 Shell, triangular, gibbous inflated, rayed, gaping ; anterior 

 slope, flattened, ribbed, cancelate ; inside, white. 



* Because it is young. Mr. Barnes. — Ed. 



