58 MOLLUSCA. 
tide with axe and chisel. According to Moseley, they 
attain an age of one hundred years. 
NotTrE.—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 Tridacne are often used as denetiers for holding holy 
water. The natives cut the shell up into knives. 
Razor-Shells (So/enidz).—This shell (Fig. 58, 2) 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. 
Fic, 61.—Pholas imbedded in a block of granite, showing section. 
Boring Shells (Po/adide).—These have a thin, brit: 
tle, and very hard shell, open at both ends, and armed witk 
file-like markings. The siphons are very long, and united 
