BIVALVES WITH SIPHONS. 57 
the winter and early spring. They live for ten or fifteen 
years. 
VALUE.—Unios produce pearls, and in St. Clair County, Illinois, 
and Rutherford County, Tennessee, their collection is a profitable 
business. In Scotland, $50,000 worth of fresh-water pearls have been 
taken from unios during the summer. A pearl was taken from a 
unio near Salem, New Jersey, a few years ago, that sold in Paris for 
$2,000. 
BIVALVES WITH SIPHONS. 
Tridacna (7ridacnide).—In the Tridacna gigas (Fig. 
59), the largest living bivalve, the shells are often five feet 
SCALE IN FEET. 
Fic. 59.—Giant clam (7ridacna gigas). 
tong ; each valve weighing over 250 pounds, the animal 
itself frequently 30 pounds, one serving as a meal for fifty 
men. The shell is trigonal, 
with deep radiations. They 
are common in the Torres 
Straits, where they are sunk 
Fic. 60.—Bivalve, with siphons. «, into the coral rock, present- 
excurrent ; 4, incurrent ; ¢, foot. ing the appearance of huge 
elongated sea-anemones, the 
mantle being of brilliant blue and green. So securely are 
they imbedded that they have to be quarried out at low 
