176 UNIONID^. 



Mr. Lea regards onr slicll as identical with the European Mtja 

 margaritifera of the older authors, the Unio elong-ata of Lamarck, 

 <fcc. ; 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 strengthen 

 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 peai'l-mussel, in which pearls of con- 

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

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

 species of fresh-water mussels. 



Margaritana undulata. 



Fig. 76. 



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

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

 valve, supported by a strong internal rib, 



Unio undnlatiis, vSat, Nicholpou's Encyc. (Amer. cd.) iv. pi. 3, fig. 3 (181G). — Bixxey's 

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



Alasmndoiita undulata, Bxrses, Silliman's Joiu-n. vi. 279 (1823). — Adams, Shells of Ver- 

 mont, in Thonips. Hist. 16.5. 



il/ya inidiiiata, Wood, Suppl. pi. 1, fig. 5. 



Muri)itntana wididata, Lea, Trans. Amer. Phil. Soc. (new series) vi. 135; Synops. Naiad. 

 44 ; 3d ed. 42. 



Unio liians, Valexc. TJecueil d'Ohs dc Zool. par Ilumh. et Bonpl. ii. 235, pi. 54, fig. 2. 



Alasmodon undula/a, Swainson, Lardner's Cab. Cyclop, cxxiii. 288, fig. 61. 



atropJutus scnIptiUs, Stimpsox, Shells of New England, 15 (1851). — The young, Lea. 



Sliell transversely-ovate, strong, much inflated, widely gaping; 

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

 four large, concentric, oblique 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 tlie ligament, and pointed at tip, rapidly narrowed ; liga- 

 mentary area imperfectly marked by an ill-defined ridge, which is 

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

 Surface a good deal undulated by the stages of growth ; epidermis 

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



