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FIRST BOOK OF ZOOLOGY. 



ductor impression. This mark is called the sinus, and, when 

 present, indicates the siphons to be of considerable size, and 

 having large muscles to contract them. 



Let these parts now be marked with a pen upon the shell, 

 with their names, as in Fig. 41 : 



Fig. 41. — Eight Valve of a Common Clam.— I, Ligament; aa, Anterior Adductor 

 Impression ; pa, Posterior Adductor Impression. 



40. On opening the shell, a prominent tooth is seen 

 on one of the valves near the hinge, while on the other 

 valve there is a corresponding depression. When the valves 

 are forcibly separated, there is left attached either to the 

 tooth, or in the depression, a substance resembling dark glue, 

 very elastic, and firmly attached to its place. This is the 

 ligament, and is said to be internal, because it is within the 

 shell, and not upon the outside, as in the fresh-water mussel 

 already studied. When the animal closes the shell by con- 

 tracting the adductors, the ligament is compressed, by being 

 jammed between the prominent tooth, and the recess into 

 which it fits. When the muscles relax, the ligament expands, 



