LAMELLIBRANCHIATA. 2 1 5 



nearly equilateral, the line drawn from the beaks to the base 

 dividing the shell into two almost equal halves. 



i 



Fig. 167. Left valve of Cytherea chione (after Woodward). A, Anterior margin : 

 B, Posterior margin ; C, Ventral margin or base ; ti Umbo ; h Ligament ; / Lunule ; 

 c Cardinal tooth ; tt Lateral teeth ; a Anterior adductor ; a Posterior adductor; / Pal- 

 lial line ; j Pallial sinus, caused by the retractor muscles of the siphons. 



The following are the chief points to be noticed in connec- 

 tion with the shell of any Lamellibranch : Each valve of the 

 shell may be regarded as essentially a hollow cone, the apex 

 of which is turned more or less to one side ; so that more of 

 the shell is situated on one side of the apex than on the other. 

 The apex of the valve is called the " umbo," or " beak " (fig. 

 167, u], and is always turned towards the mouth of the animal. 

 Consequently the side of the shell towards which the umbones 

 are turned is the " anterior" side, and it is usually the shortest 

 half of the shell. The longer half of the shell, from which the 

 umbones turn away, is called the " posterior" side, but in some 

 cases this is equal to, or even shorter than, the anterior side. 

 The side of the shell where the beaks are situated, and where 

 the valves are united to one another, is called the " dorsal " 

 side ; and the opposite margin, along which the shell opens, is 

 called the " ventral " side, or " base." The length of the shell 

 is measured from its anterior to its posterior margin, and its 

 breadth from the dorsal margin to the base. 



At the dorsal margin the valves are united to one another 

 for a shorter or longer distance, along a line which is called the 

 " hinge-line." The union is effected in most shells by means 

 of a series of parts which interlock with one another (the 

 " teeth "), but these are sometimes absent, when the shell is 



