MOLLUSCA. LAMELLIBRANCHIA 203 



movement of the foot (protractors and retractors) ; these 

 occur close to the anterior and posterior adductors. 

 Passing from one adductor impression to the other in 

 each valve is a linear depression, caused by the attach- 

 ment of the muscles of the mantle to the shell, and known 

 as the pallial line (pi). In some forms this line runs 

 evenly between the two adductor impressions and parallel 

 with the margin of the valve ; it is then said to be simple 

 or entire. But in those genera which possess retractile 

 siphons the pallial line bends inward just before reaching 

 the posterior adductor ; this indentation is known as the 

 pallial sinus (s), and is caused by a part of the pallial 

 muscles which serve for the retraction of the siphons. 



The hinge is formed by projections known as teeth, 

 which alternate in the two valves, the teeth of one valve 

 fitting into the depressions between those of the other. 

 The margin of the valve on which the teeth occur is 

 known as the hinge-line; generally it is curved, but in 

 some genera it is straight (e.g. Area). Several types of 

 hinge may be recognised: — (1) Taxodont : the teeth are 

 numerous and more or less similar in form and size, 

 e.g. Nucula (fig. 93 A). (2) Dysodont : the teeth are of a 

 simple type, and are developed from internal ribs at the 

 margin of the valve ; the hinge-margin may be simple or 

 somewhat thickened, e.g. Mytilus. (3) Isodont : there are 

 two strong teeth of equal size in each valve, which fit into 

 corresponding sockets in the other valve ; between the 

 teeth is a median ligament-pit, e.g. Spondylus (fig. 93 D, E). 

 (4) Schizodont : the teeth are few in number, thick, and 

 sometimes grooved ; the middle tooth in the left valve is 

 often bifid, e.g. Trigonia (fig. 93 B, C). (5) Heterodont : the 

 teeth are few in number and not all of uniform shape and 

 size ; some (usually two or three) are placed immediately 



