THE largest bivalves in the world belong to the Tridacnicte. They have always aroused much interest in conchologists although little was known of the living animals. Size is not the only distinction in the family, for Prof. C. M. Yonge shows that they are unique among the Lamellibranchiata in the relation of the mantle and shell to the other organs, and in the universal presence of zooxanthellse in the tissues. In his peculiarly interesting monograph, there is a large amount of new matter clearing up much that was puzzling in these gigantic molluscs, which are among the most conspicuous members of the fauna of coral reefs in the Indo-Pacific region. Tridacna derasa, the giant clam, may be 4J ft. long: the largest lamellibranch ever evolved, and may weigh about 4 cwt. The largest specimens personally examined were a little more than 3 ft. in length, and were so heavy that the combined efforts of two men failed to raise them.