344 The American Naturalist. [April, 
to me not improbable in certain cases, where water and other 
elements of environment appear to be favorable for producing 
normal conditions of the hinge, that the fact of this part of the 
shell being nearly always blurred and distorted goes to show 
that the diseased condition has become more or less fixed and 
is inherited. 
Be this as it may the evidence of the shells and soft paté 
seems to show clearly that Margaritana is not a valid genus, 
but that the name merely stands for certain groups or parts of 
groups of Unios of polyphyletic origin, and that all the species 
will have to be relegated to the genus Unio. 
