MARGARITANA. 



177 



tcrnately yellowish and dark, which are not very conspicuons nulcss 

 hold up to transmitted light. Within, the anterior half is thick- 

 ened, opaque, and the color 

 wliite ; the posterior half is 

 translucent, thin, of a silvery 

 lustre, exhibiting the exterior 

 radiations. Hinge supported 

 on a very strong rib, tooth of 

 the right valve erect, conical, 

 striated above ; tooth of the 

 left valve erect, produced back- 

 wards in a triangular manner, 

 under the ligament, with a 

 pit in front of it; cavity of the beaks very deep and capacious. 

 Length, two inches ; height, one and two fiftlis inches ; breadth, 

 one inch. 



Found in Blackstone River and its tributaries, and in Plymouth 

 County. 



It is easily known by its short, tumid appearance, the undulations 

 on the beaks, and the peculiar hinge. 



M. umhiiata. 



Margaritana marginata. 



Fig. 77. 



Shell transversely-ovate, wedge-shaped, inequilateral; beaks prominent; nm- 

 bonal ridge elevated ; surface wrinkled posteriorly ; epidermis olivaceous, imper- 

 fectly radiated with dark green; tooth small, compressed, looking forwards; 

 nacre bluish-white, with a chalky-white margin. 



Alasmodonfa margmata, Say, Journ. Acad. Nat. Sc. i. 459. — Barxes, Silliman's Journ. 



vi. 279. — Co\RAD, Synops. in Proc. Ac. Nat. Sc. vi. 262. — Stimpson, Shells of 



Now England, 1.5. — Adams, Shells of Vermont, in Thomps. Hist. 165. 

 Unh varicosa. Lam. An. sans Vert. vi. 78 ; 2d ed. vi. 54-3. 

 Alnsmndon marginata, De Kay, Nat. Hist. New York, 196, pi. 14, fig. 225. — Gould, 



Inv. 116, fig. 77. 

 Margan'taia marginata, Lea, Trans. Amcr. Phil. Soc. (new series) vi. 135; Synops. .3d 



ed. 42. 

 Mij(t rugidosa, Index, pi. 1, fig. 7. 



Shell ovate, thin, widely gaping behind, wedge-shaped from lie- 

 fore backwards ; beaks at the anterior third, rather small, but ele- 

 vated, and having three or four small undulations ; anteriorly low 

 and rounded, but increases rapidly in height ; the posterior hinge- 

 margin suddenly declines to form a rounded tip ; ridge from the 

 12 



