MARGARITANA. 



177 



tcrnatcly yellowish and dark, wliicli are not very conspicuous unless 

 held up to transmitted liglit. Within, the anterior half is thick- 

 ened, opaque, and the color 

 white ; the posterior half is 

 translucent, tliin, of a silvery 

 lustre, exhibiting: the exterior 

 radiations. Hinge supi)orted 

 on a A'cry strong rib, tooth of 

 the right valve erect, conical, 

 striated above ; tooth of the 

 left valve erect, produced hack- 

 wards in a triano:ular manner, 

 under the ligament, with a 

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

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

 one inch. 



Found in Blackstone River and its tributaries, and in Plymouth 

 County. 



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

 on the beaks, and the peculiar hinge. 



M. tinilulata. 



Margaritana marginata. 



Fig. 77. 



Shell transversely-ovate, wedpfe-shaped, inequilateral ; beaks prominent ; um- 

 bonal ridge elevated; surflxce wrinlvled posteriorly ; epidermis olivaceous, imper- 

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

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



Alasmodonta viarginnta, Say, Joiirn. Acad. Nat. So. i. 450. — Barnks, Silliman's Jonrn. 



vi. 279. — CoxRAD, Synops. in Proc. Ac. Nat, Sc. vi. 262. — Smipsox, Shells of 



New England, 1."). — Adams, Shells of Vermont, in Thomps. Hist. 1G.5. 

 Unio iKiricosa, Lam. An sans Vert. vi. 78 ; 2d ed. vi. 543. 

 Alasmodon manjinata, De Kay, Nat. Hist. New York, 196, pi. 14, tig. 225. — Gould, 



Inv. 116, fig. 77. 

 Marfimitiina marginata, Lea, Trans. Amer. Phil. Soe. (new series) vi. 135; Synops. 3d 



ed. 42. 

 Mija rmjidosa, Index, pi. 1, tig. 7. 



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

 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 



