PELECYPODA. 



827 



and left valves can generally be named by observation of the beaks 

 and ligament. When the ligament is median to the valves (Ostrea, 

 Pecten], and the beaks are not curved, the valves may be recognised 

 by noting the fact that the impression of the adductor muscle (in these 

 cases always single) is nearer to the posterior than to the anterior 

 side. In a similar way the pallial sinus, which is always posterior, 

 furnishes a guide. In the fixed inequivalves it is sometimes the right, 

 sometimes the left valve which is undermost ; but the fixed valve is 

 always deep, and the free valve flat. Ostrea and Anomia are always 

 fixed by the left valve. 



es 



FIG. 1. LEFT VALVE OF VENUS. 



A, anterior ; B, posterior ; C, dorsal ; D, ventral, margins. AB, length ; CD, 

 breadth, of shell, am, anterior ; pm, posterior, adductor muscle ; p, pallial 

 line ; ps, pallial sinus ; I, ligament ; lit, lunule ; u, umbo ; c, cardinal teeth ; 

 al, anterior ; pi, posterior, lateral tooth. 



FIG. 2. 

 es, escutcheon ; li, ligament ; lu, lunule, ; u, u,, umbones. 



FIG. 3. HINGE OF RIGHT VALVE (A) AND LEFT VALVE *(B) OF MTJLINIA. 

 co, cardinals ; la, anterior ; Ip, posterior, laterals ; /, fossette ; c, resilium, or 



cartilage ; I, ligament. 



The lunule is a well-marked area in front of and close to the um- 

 bones, usually more or less heart-shaped, and limited by a ridge. A 

 corresponding area behind the umbones, enclosing the ligament, is 

 called the escutcheon, but it seldom occurs. 



The ligament proper is of a more or less horny nature, and, as the 

 most important factor in the mechanism cf the valves, has undoubtedly 

 developed with the evolution of the class, and its chief modifications 

 date from the earliest period in the life-history of the group. The 

 function of the original ligament was that of an external link between 

 the valves having the essential nature of a C spring that is, the 

 insertion of the ligament-edges on the cardinal margins, or, at a later 

 period, on thickened ridges or nymphcB, by which these margins are 

 reinforced to bear strains, resulted in the following conditions : The 



