332 ELEMENTARY BIOLOGY. [CHAP. 



and an anterior larger and posterior smaller end, 

 the latter compressed dorso-laterally note the soft 

 chitinous ventral edge of each valve. 



b. The umbo; a small blunt eminence on the 

 dorsal border of each valve near its anterior 

 end; its apex is directed forwards. 



Note the delicate texture of this area. 



c. The ligament ; an elastic uncalcified part of the 

 exoskeleton behind the umbones, uniting the two 

 valves and tending to divaricate their ventral edges. 



d. External markings. The outside of the shell is 

 greenish brown, and on it are seen a number of 

 concentric lines of growth, running parallel to the 

 margin of the shell and more numerous towards 

 its ventral edge. 



e. Detach one valve by cutting through the ligament 

 with a scissors; before removing the animal 

 examine under water, and note the relations of the 

 chitinous ventral edge (a.). It extends along the 

 straight dorsal border of the valve in front of the 

 ligament ; and from that point in front, as from 

 the posterior edge of the ligament behind, it is 

 sharply inflected passing inwardly to be reflected 

 on to the free border of the pallium. (Cf. Sect. 

 L. a. y.) 



/ Internal markings. The interior of the valve is 

 white and iridescent : on it are seen, near the dorsal 

 border, two oval scars, the anterior and posterior 

 adductor impressions. 



Joining the two adductor impressions is a double 

 curved line, the pallial impression, which marks the 

 point of attachment of the pallial muscle. 



