182 THE AMERICAN NATURALIST. 
of common origin ; the few-toothed ledas approach the hetero- 
dont type, sometimes very closely; but the divergencies must 
have begun very early in the history of pelecypods and the 
differentiation is now very marked. It is earnestly to be 
hoped that such researches as those of Bernard will be con- 
tinued until light is thrown on all the various problems for 
which we now know no suggestion of a solution. 
UNITED STATES NATIONAL MUSEUM. 
