52 



in the veliger and the velum and foot in the veliconcha, are situated in the 

 anterior margin. The opposite end, where siphons later develop, is called the 

 posterior. Morphologically, the valves join on the dorsal margin. Since in 

 larvae the hinge develops along the dorsal margin, this margin is also termed 

 the hinge margin. The margin opposite to the hinge is called the ventral 

 margin. The anterior part of the dorsal margin (up to the umbo) is called the 

 anterior shoulder, and the posterior part (from the umbo), the posterior shoul- 

 der (Figure 34). 



Sculpturing, concentric (lines of growth) and radial lines appear on the 

 outer surface of the valves. In individual species the pallial line, the imprint 

 of the mantle muscle, is quite noticeable on the inner surface (Figure 34). 



The hinge in larvae consists of a provinculum and a lateral system (see 

 Figure 18). The provinculum is a thickening of the hinge margin and bears 

 cardinal teeth. The lateral hinge system is represented by flanges and crests 

 (Rees, 1950; Zakhvatkina, 1959). In a closed shell the thicker dorsal margins 

 of the flanges of one valve are overlapped by the thinner dorsal margins and 

 iimer crests of the other valve. Sometimes lateral teeth develop on the flanges 

 where their ends touch the provinculum. In some species there are special 

 teeth, which belong neither to the provinculum nor to the lateral hinge system. 



pa II 



Figure 34: Schematic structure of the larval shell in different stages. 



A — veliger; B — vehconcha; C — spat; D — veliconcha. 



am — anterior margin; as — anterior shoulder; d — shell thickness; dis — dissoconch; 



dm — dorsal margin; gl — growth line; h — shell height; hi — hinge; 1 — shell length; 



lig — ligament; pd 1 — prodissoconch I; pd II — prodissoconch 11; pm — posterior margin; 



ps — posterior shoulder; u — umbo; vm — ventral margin. 



