108 INVERTEBRATA OF MASSACHUSETTS. 



pearl tinged violet. Hinge having a single, erect, pyramidal, 

 coarsely striated cardinal tooth in the right valve, with the vestige 

 of a tooth before, and a pit behind it ; on the left valve are two 

 nearly equal teeth of a triangular, pyramidal form, the space be- 

 tween them corresponding to the opposite tooth ; lateral teeth 

 compressed, long, very slightly curved. Ordinary length 3^ in- 

 ches, height 2 inches, breadth 1 inch. 



This is the most common fresh-water mussel we have. It is 

 found in every considerable brook or collection of water emptying 

 into the Atlantic ; and it is said never to be found in any of the 

 streams beyond the Atlantic slope. 



It is perhaps the most variable of all species, as we may judge by 

 noticing the numerous species, which, according to Mr. Lea, Lamarck 

 made of its varieties. And it is to be feared that Mr. Lea himself has 

 not entirely avoided this error. Certain it is, that shells answering 

 well to his Roanokensis, jejunus, and some others, are not seldom found 

 in Massachusetts, among the indisputable complanatus. 



The shell is always rounded before, somewhat widened and angular 

 behind, and slightly truncated at tip, especially if viewed inside. Nor 

 is it ever much inflated. Its true form is transversely oblong-oval ; 

 but it is often nearly oval, and sometimes is much curved, In one speci- 

 men before me, the height is five eighths of the length, and in another it 

 is only two fifths. In this latter, the breadth is one fourth of the length, 

 while, in a third, it is nearly one half. The epidermis is usually coarsely 

 wrinkled, without lustre, and of a pitchy-black color; but I have 

 a series of a strongly marked variety from the Shawsheen river in 

 Andover, where the color is dark-chestnut with considerable lustre, 

 the young shells are radiated with dark lines almost as much as U. 

 radiatus, and some of the^old shells are very coarsely plaited, from the 

 beaks downwards, into parallel folds. 



The only New England species with which this is liable to be con- 

 founded are, U. nasutus and U. radiatus. The first differs, exter- 

 nally, in its more smooth, greenish, and somewhat radiated epidermis ; 

 the angular ridge running from the beaks backwards, produced by the 

 strong compression of the hinge margin ; and by a contraction of the 

 basal margin, near its posterior termination, so as to form a sort of 

 beak ; and internally by the silvery, iridescent nacre, and the slender, 

 very oblique cardinal teeth, U. radiatus has the hinge very nearly 

 the same as U. complanatus ; but the nacre is white, or somewhat 



