178 



unionidj:. 



Fie;. 479. 



J\I. mnrginata. 



beaks elevated and well-defined, above which the shell exhibits 

 coarse, rounded wrinkles, running obliquely upwards and back- 

 wards ; eipdermis shining, 

 olive-green, somewhat mot- 

 tled with dark and light 

 shades, and with oliscure, 

 broken, radiating lines ; 

 within bluish-white, with 

 shades of green, the mar- 

 gin chalky-white. Hinge 

 delicate, the teeth, one in 

 each valve, small, com- 

 pressed, directed along the 

 hinge-margin so as almost to coincide with it ; sometimes the teeth 

 are only rudimentary ; cavity of the beaks rather deep, not very ca- 

 pacious. Length, two inches ; greatest height, one and one tenth 

 inches ; breadth, nine tenths of an inch. 



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

 shin River, Andover. I have also received very beautiful specimens 

 from a pond in West Brookfield. 



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

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

 markable 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 M. rng-osa. It is 

 more elongated than M. 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 M. trimcata by Say. Some of our specimens approach it 

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



Oenus ANODOX, (Brug.) Cuvier. 1798. 

 Shell transversely elongated, inequilateral, thin ; hinge toothless. 



Anodon fluviatilis. 



Fig. 80. 



Shell thin, inflated, transversely sub-oval, hinge-margin straight, crested be- 

 hind ; beaks moderately elevated, epidermis deep grass-green, obscurely rayed, 

 darker above the posterior ridge : within white, tinted lilac. 



