204 



MOLLUSCA. LAMELLIBRANCHIA 



under the umbo and are known as the cardinal teeth, 

 others, termed laterals, are placed in front of and behind 

 the umbo, forming the anterior and posterior laterals ; 

 some or all of the cardinals or of the laterals may be 



-c'^'WBj 



Fig. 93. Some types of hinge. A, Nucula. a, anterior adductor ; 

 b, posterior adductor ; I, ligament-pit. B, C, Trigonia. B, right 

 valve with two large striated teeth ; C, left valve with three teeth. 

 D, E, Spondylus ; D, right valve ; E, left valve ; a, b, teeth ; c, d, 

 sockets into which the teeth fit; e, area; I, ligament-pit. F, Lucina 

 (right valve) ; a, anterior lateral tooth ; b, cardinal tooth ; c, pos- 

 terior lateral tooth ; /, ligament. G, Lutraria (left valve) ; a, strong 

 A-shaped cardinal tooth; /, process to which the ligament is attached. 

 All drawn from recent specimens. 



absent; the hinge-margin is usually extended as a vertical 

 lamina or hinge-plate (fig. 93 F) on which the teeth are 

 borne, e.g. Meretrix. (6) Desmodont : true teeth and a 



