176 UNIONID^. 



Mr. Lea regards our shell as identical with the European Mya 

 margaritifera of the older authors, the Unio cJongata of Lamarck, 

 &c. ; but the shells which I have had an opportunity of examining 

 present some constant differences. The foreign shell is shorter, the 

 beaks more nearly central, and more elevated, and the portion of 

 the interior, within the pallial impression, is minutely granulated ; 

 and as my foreign specimens agree accurately with the figures of 

 Chemnitz and Turton, I am induced to think there may be a con- 

 stant difference. The intervention of an ocean would stren»'then 

 the supposition. I have, therefore, felt disposed to retain the very 

 appropriate name of Barnes, until more fully satisfied. The Euro- 

 pean shell is the famous river pearl-mussel, in which pearls of con- 

 siderable beauty arc occasionally found. But, as far as I have ob- 

 served, they are not oftener found in our Alasmodori than in other 

 species of fresh-water mussels. 



Margaritana undulata. 



Fig. 76. 



Shell trausversely-ovate, inequilateral, angular behind ; beaks tumid, elevated, 

 undulated ; epidermis durk-green, obscurely rayed ; one cardinal tooth in each 

 valve, supported by a strong- internal rib. 



Umo iindulatus, Say, Nicholson's Encyc. (Amer. cd.) iv. pi. 3, fig. 3 (1816). — Bixney's 

 reprint, 53, pi. 71, fig. 3. 



Alasmodonta undulata, BARXsiS, Silliman's Journ. vi. 279 (1823). — Adams, Sliclls of Ver- 

 mont, in Thonips. Ilist. 16.5. 



Mya undulata, Wood, Suppl. pi. 1, fig. 5. 



Margaritana undulata, Lea, Trans. Anicr. Phil. Soc. (now series) vi. 135; Synops. Naiad. 

 44 ; 3d ed. 42. 



Unio Mans, Valenc. Recncil d'Obs dc Zool. par Ilunih. et Bonpl. ii. 235, ]>!. 54, fig. 2. 



Alasmodon undnlata, Swainsox, Lardncr's Cab. Cyclo]). cxxiii. 288, fig. 61. 



iStrophitus scti/ptilis, Stimpsox, Shells of New England, 15 (1851). — The young. Lea. 





Shell transversely-ovate, strong, much inflated, widely gapin 

 beaks at the anterior third, very prominent, tumid, with three or 

 four large, concentric, ol.)liquc undulations upon them ; anterior and 

 basal margins broadly and regularly curved, with a very slightly 

 lobed appearance in front of the beaks ; posterior margin angular 

 behind the ligament, and pointed at tip, rapidly narrowed ; liga- 

 mcntary area imperfectly marked by an ill-dclined ridge, which is 

 usually wrinkled in the direction of its course ; margin compressed. 

 Surface a good deal undulated l)y the stages of growth; epidermis 

 shining, of a dark olive-color, everywhere rayed with fine lines, al- 



