NAIADES. MOLLUSCA. ALASMODON. 115 



have, therefore, felt disposed to retain the very appropriate name of 

 Barnes, until more fully satisfied. The European shell is the famous 

 river pearl-mussel, in which pearls of considerable beauty are occa- 

 sionally found. But, as far as I have observed, they are not oftener 

 found in our Alasmodon than in other species of fresh-water mus- 

 sels. 



ALA'SMODON UNDULA^TA. 



Sliell 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. 



FIGURE 76. 



State Coll., No. 177. Soc. Cab., No. 2174. 



U^nio undulata, SAY ; Nicholson's Encyc., (Amer. ed.), iv. pi. 3, f. 3. 



Alasmodonta undulata, BARNES; Silliman's Journ., vi. 279. 



My a undulala, WOOD ; Suppl., pi. 1, f. 5. 



Margaritana undulata; LEA ; Trans. Am.tr. Philos. Soc., (New Series,) vi. 135. 



Synops. Naiad., 44. 

 tPnio hians, VALENC. ; Recueil d'Obs. de Zoo/., par Humb. et Bonpl. ii. 235, pi. 



54, f. 2. 

 Alasmodon undulata, SWAINSON; Lardner's Cab. Cyclop., cxxiii. 288, f. 61. 



Shell 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 the ligament, and pointed at tip, rapidly narrowed ; 

 ligamentary 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, alternately yellowish and dark, which are 

 not very conspicuous unless held up to transmitted light. Within, 

 the anterior half is thickened, opaque, and the color white ; the 

 posterior half is translucent, thin, of a silvery lustre, exhibiting 

 the exterior radiations. Hinge supported on a very strong rib, 



