286 Vr. Hildreth on fresh water shells. 



and I believe is a faithful representation of the individual de- 

 signed. The inner surface of some shells, is sprinkled over 

 with small grains, like mustard seed in size, while others are 

 nearly or quite free from them, as is the case with the present 

 shell.* 



No. 18. Unio Prjelongus. — Fig. 18. 



Shell much elongated transversely, narrow, thick, tumid, 

 beaks flat, lateral tooth long, thin ; inside white, tinged with 

 green or purple. 



Length, 2.25 ; breadth, 5.& ; diameter, 1.9. — Hab. Duck 

 Creek. 



Shell, very long oval ; anterior side somewhat pointed ; 

 posterior side short rounded, obtuse ; beaks depressed ; liga- 

 ment elevated above the beaks ; basal margin slightly com- 

 pressed ; when young, rounded ; epidermis, blackish brown, 

 wrinkled transversely, and rayed obscurely ; naker, white, 

 and tinged with spots of green, or purple under the beaks, 

 with a row of small muscular impressions in the cavity ; pos- 

 terior cicatrix deep and not very rough ; iridescent on the 

 forepart. 



No. 19. Unio Gibbosus. — Fig. 19. 



Shell, elongated transversely, thick and gibbous ; later 

 tooth thick, incurved, inside purple. 



Length, 2.00 ; breadth, 4 ; diameter, 1,1.00. — Hab. Musk- 

 ingum. 



Shell, much elongated transversely, thick and heavy ; ra- 

 pidly narrowed and rostrate before, narrow and rounded be- 

 hind ; disks somewhat compressed ; anterior side much pro* 

 duced ; beaks flat ; ligament elevated ; anterior dorsal mar- 

 gin, depressed and flattened ; basal margin, nearly strait ; 

 epidermis, dark brown, deeply wrinkled transverely ; naker, 

 purple ; teeth, crenate ; lateral tooth, thick and rough, and 

 folded over towards the inside of the shell ; posterior cicatrix, 

 deep, and rather rough ; so deep that in old specimens, it is 

 often worn through on this part. 



No. 20. Unio Radiatus. — Fig. 20. 

 Shell, ovate, thin, finely striated, glossy, rayed, within blu- 

 ish white.! 



* We are informed by Mr. Barnes, that they may be procured in abundance at 

 Ticonderoga. — Ed. 



t In Mr. Barnes's opinion, a young Ventricosus, and not the true Radiatus 



—Ed. 



