NAIADES. MOLLUSCA. ANODON. 117 



beaks rather deep, not very capacious. Length 2 inches, greatest 

 height 1-iV inch, breadth & inch. 



Found in the Blackstone River and its tributaries, and in Shaw- 

 shin River, Andover. I have also received very beautiful speci- 

 mens from a pond in West Brookfield. 



It not common, and may be readily distinguished from our other 

 species by its wedge-like form, when seen from above, by the remark- 

 able series of oblique wrinkles along the posterior slope, and by its 

 delicate teeth, which, in fact, sometimes wholly disappear. In the 

 character of its wrinkles it is much like A. rugosa. It is more 

 elongated than A. undulata, and has its greatest height at the posterior 

 termination of the hinge, instead of opposite the beaks, as in that shell. 



Mr. Lea regards our shell as being the same as the western shell 

 named A. truncdta by Say. Some of our specimens approach 

 them very closely, but ours is in general a less inflated, less angular 

 shell. 



GENUS NODON, BRUG. 



Shell transversely- elongated, inequilateral, thin ; hinge toothless. 



i 

 ANODON FLUVIATILIS. 



Shell thin, inflated, transversely sub-oval, hinge margin 

 straight, crested behind ; beaks moderately elevated, epidermis deep 

 grass-green, obscurely rayed, darker above the posterior ridge ; 

 within white, tinted lilac. 



FIGURE 80. 

 State Coll., No. 171. Soc. Cab., No. 2181. 



Anoddnta cataracta, SAY ; Nicholson's Encyc., (Amer. ed.), iv. pi. 3, f. 4. 



My'tilus fluviatilis, DILLWYN; Gated. 



Anodonta fluviatilis, LEA; Trans. Amer. Phil. Soc., (New Series,) vi. 138. 



Synops. Naiad., 51. 



My'tilus illitus, SOLANDER; Portland Catal., 163. 

 LISTER; Conch., t. 157, f. 12. 



Shell transversely sub-oval, sub-cylindrical, thin, fragile, in- 

 flated ; beaks at the anterior two fifths of the shell, tumid, some- 

 what elevated, and minutely undulated at tip. Hinge margin 

 straight ; anterior imperfectly angular above, nearly as high as be- 



