116 INVERTEBRATA OF MASSACHUSETTS. 



tooth of the right valve erect, conical, striated above ; tooth of 

 the left valve erect, produced backwards in a triangular manner, 

 under the ligament, with a pit in front of it ; cavity of the beaks 

 very deep and capacious. Length 2 inches, height If inch, 

 breadth 1 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. 



Alasmodon margina'ta. 



Shell transversely-ovate, icedge-shaped, inequilateral ; beaks 

 prominent ; surface obliquely wrinkled posteriorly ; epidermis 

 olivaceous, imperfectly radiated with dark green ; tooth small, 

 compressed, looking forwards ; nacre bluish-white, icith a chalky- 

 white margin. 



Figure 77. 



State Coll., No. 173. Soc. Cab., No. 2171. 



Alasmodonta marginata, Say ; Journ. Jlcad. jYat. Sc, i. 459. Barnes ; Silliman's 



Journ., vi. 279. 

 Unio varicosa, Lam. ; ^n. sans Vert., vi. 543. 

 Alasmodonta truncata ? Sav. 



Margaritana marginata, Lea ; Trans. £mcr. Philos. Soc, (New Series,) vi. 135. 

 Mya rugulosa, Wood ; Index, SuppL, pi. 1, f. 7. 



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

 before backwards ; beaks at the anterior third, rather small, but 

 elevated, 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 beaks elevated and well defined, above which the shell exhibits 

 coarse, rounded wrinkles, running obliquely upwards and out- 

 wards ; epidermis shining, olive-green, somewhat mottled with 

 dark and light shades, and with obscure, broken, radiating lines ; 

 within bluish-white, with shades of green, the margin chalky- 

 white. Hinge delicate, the teeth, one in each valve, small, com- 

 pressed, directed along the hinge margin so as almost to coincide 

 with it ; sometimes the teeth are only rudimentary ; cavity of the 



