22 WEST COAST SHELLS 



a gulf, or bay. If the creature simply lives on the 

 surface of the mud, or is only partly buried in the 

 sandy bottom, he will have no need for long pipes, 

 and the pallial sinus will be omitted. 



The two big scars marked a and a' show where 

 the adductor muscles are grown to the shell. These 

 muscles are strong affairs, and enable the creature 

 to close his doors with a snap and to hold them 

 closed against most intruders. Sometimes there is 

 but one adductor, as in the oyster, and then it is situ- 

 ated near the center of the shell. But when the clam 

 or the oyster shuts his shell he is obliged to stretch 

 a thick, heavy spring, which, when it is wet, resem- 

 bles rubber; though it gets brittle when it is dry. 

 This spring is marked "1" in the picture, which 

 stands for ligament. In some species the ligament is 

 internal instead of external, and in such cases it is 

 compressed instead of lengthened, when the clam 

 closes his shells. In either case the elastic ligament 

 opens the valves as soon as the muscles become re- 

 laxed, either by the will of the clam or by his death, 

 for dead shells are nearly always open. 



The ligament also serves to keep the two valves 

 from falling apart, and beside it there are more or 

 less hinge-teeth for the same purpose. The lateral 

 tooth is marked "1. t." in the picture, while the three 

 cardinal teeth are situated just below the umbo, 

 which is marked "u." One more mark, namely, 

 "lu," signifies lunule. This is a heart-shaped depres- 

 sion on the outside of the shell, half in each valve, 

 and is seen best when you look at the end of the shell, 

 as in Figure 54. 



