58 MOLLUSC A. 



tide with axe and chiseL According to Moseley, they 

 attain an age of one hundred years. 



NOTE. So powerful are they that large sharks and rays that have 

 accidentally crossed them have been seized and held. The Tridacna 

 always harbors within its shell several crabs. The byssus is so large 

 that it can only be cut with a hatchet. Eight species are known. 



VALUE. The Tridacnse are often used as benetiers for holding holy 

 water. The natives cut the shell up into knives. 



Razor-Shells (Solenidtz). This shell (Fig. 58, ) is 

 long, thin, and slightly curved, with two or three teeth in 

 each valve. They have a powerful club-shaped foot, do 

 not secrete a byssus, but lie concealed upright in the sand. 

 When placed upon the sand, they use the foot like an au- 

 ger, and rapidly disappear ; 60 or more living species are 

 known, and 350 fossil. 



FIG. 61. Pholas imbedded in a block of granite, showing section. 



Boring Shells (Pholadidofy, These have a thin, brit- 

 tle, and very hard shell, open at both ends, and armed with 

 file-like markings. The siphons are very long, and united 



