170 CO^CHYLIA—DITHYRA. 39. 



fringes the margin : beaks distant, with a black striate ob- 

 long space between them ; inside blueish-white, ribbed longi- 

 tudinally. 



39. FECTU^CVLVS—LA31ARCK. 



Testa suborbicularis, fequiKalvis, snbfequilateralis, clausa ; 

 mnbonibiis conniventibus, ataie yradatvm remotioribus. 

 Cardo arcuatus, dentibus numerosis obtusis alternatim in- 

 sertis, centralibiis obsoletis -• laleralibus nullis. Ligamen- 

 tum externum. 



Shell mostly orbicular, equivalve, generally equilateral, closed ; 

 the beaks close together, but becoming gradually more remote 

 by age. Hinge in a curved line, with numerous obtuse teeth 

 alternately closing together, the central ones obliterated : 

 lateral teeth none. Ligament external. 



The rounded outline of this genus, causing the hinge to form 

 the segment of a circle, instead of being placed in a straight 

 line, naturally separates it from the Area. The species are all 

 inhabitants of the deeps, far more remote from the shores than 

 the Cardium and Venus tribes, and are not attached by a byssus, 

 like most if not all of the Area family. All have the inner 

 margin notched. 



