MOLLUSCA 63 



very valuable, but in America they have never been very systemati- 

 cally sought, being"mainly collected by farmers and others in a very 

 wasteful manner. In Europe the supply has been conserved by care- 

 fully opening the shells with specially devised tools and returning the 

 animals to the water after examination. 



In America the Unionidae are chiefly used for buttons and along the 

 Ohio and Mississippi Rivers they have been so extensively dredged that 

 they are threatened with extermination. The shells are drilled with 

 tube-shaped drills that cut out disks of pearl from about 5 mm. to 30 

 mm. or more in diameter, Fig. 44; from these rough disks the pearl but- 



FIG. 44. Ohio River pearl mussel, Unio sp., from which 15 button discs have been 

 cut. Above are 3 discs of different sizes. X/^. 



tons are made. Several dozen disks may be drilled from one shell, the 

 number varying, of course, with the size of the disks and the size of the 

 shell. 



In both the pearl-bearing oysters and pearl-bearing mussels pearls 

 are most likely to be found in distorted and abnormal specimens. 



Ship-worms. Of the few molluscs that are of negative economic 

 importance probably the most injurious are the ship-worms, Teredo 

 navalis and other species. In the days when all vessels were made of 

 wood and were unprotected with metal sheathing these boring bivalves 

 must have been, indeed, the ''terror of ships." Though they belong 

 to the class of Lamellibranchs they are elongated, worm-like animals, 

 and hence have been called ship-worms. They may reach a length of 



