288 Br. Hildreth on fresh water Shells. 



Length, 1.25 ; breadth, 2.00 ; diameter, 1.1. — Hab. Duck 

 Creek. 



Shell, moderately thick, acutely angulated before, obtuse, 

 and somewhat angulated behind ; disks, inflated ; anterior 

 slope, flattened, very broad, ribbed longitudinally, and wrink- 

 led transversely ; beaks, one third from posterior extremity, 

 decorticated, approximate, and somewhat elevated ; anterior 

 lunule, oval heart shaped, in the smaller, and perfectly heart 

 shaped in the larger specimens ; basal margin, a little de- 

 pressed near the anterior extremity ; anterior margin, angula- 

 ted ; posterior margin, rounded and broad ; epidermis, yel- 

 lowish green, rayed with dark green, finely striated trans- 

 versely, and with from three to six, more conspicuous trans- 

 verse wrinkles ; anterior slope, marked with longitudinal ribs, 

 which are beautifully cancelated ; ribs, projecting and form- 

 ing a dentated edge ; cardinal teeth, two in each valve, com- 

 pressed and crenulate ; lateral teeth, short, projecting, and 

 terminating abruptly ; naker, bluish white, slightly iridescent. 



No. 23. Unio Gracilis.— Fig. 23. 



Shell, ovately triangular, very thin and fragile ; hinge mar- 

 gin, elevated ; ligament, concealed. 



Length, 2.5 ; breadth, 3.5 ; diameter, 1.25. — Hab. Little 

 Muskingum. 



Beaks, depressed and placed far back ; ligament, between 

 the valves, and covered ; anterior lunule, distinct ; hinge mar- 

 gin, elevated into a large wing, in the perfect specimens ; 

 epidermis, sea green, wrinkled and striated transversely, gla- 

 brous ; cardinal teeth, very small, scarcely projecting ; later- 

 al teeth, very thin and delicate ; naker, bluish white, tinged 

 with violet, and beautifully iridescent. 



Remark. — This shell is but a small specimen of the graci- 

 lis, in these waters. I have heard of one three times the size 

 — the wing is much mutilated. The contour of the shell, in- 

 dependent of the wing, is much like that of the alatus. It is 

 a more delicate shell, and inside more beautifully irised, if 

 possible — not a common shell in this vicinity, as I have but 

 one specimen.* 



The above, are all the specimens of the Unio, that have as 

 yet fallen under my notice ; but as my researches have 



It is abundant in Lake Champlain.( D. H. B.) — Ed. 



