318 Cincinnati Society of Natural History. 



the definition be applied to a number of existences so infinitely small, 

 when compared with the living m3"riads around us, as to be absolutely 

 fatal to it b}^ contrast. 



The belief of the past, that all species are immutable productions, 

 originating from points within the limits where they are now found, 

 and which have spread from specific centers to the limits of suitable 

 conditions, their areas thus being larger or smaller, according to cir- 

 cumstances, made a reference to potential physical factors of the past 

 a necessit}^; and it required a belief in the effect of these changes, of 

 whatever nature it may have been, as a restriction upon the limits of 

 distribution. Recent reasoning but enlarges this field of view, in ac- 

 cordance with our wider information as to the capacit}^ of animals 

 for adaptation, and their proneness to. variability; these factors render- 

 ing it possible for animals to overstep an}' artificial obstructions raised 

 by the imagination, and tending to render the phrase " limit of suitable 

 conditions," an exceedingly uncertain boundar3^ 



The relations existing between species and distribution are now so 

 generally recognized, that I need say nothing farther in the way of an 

 introduction to this discussion. It would appear that all these questions 

 become of paramount importance in the stud}- of our fresh-water moll- 

 usca, because they have always been subjected to a series of causes 

 from which, on account of their peculiar station, they have been more or 

 less powerless to escape; and that, in consequence of this, they have ex- 

 hibited a high degree of capacity for adaptation, with a maximum 

 variabilit}^ of form as shown by their present development. It is, 

 therefore, a field of special interest, to which this paper briefly calls at- 

 tention. 



It seems desirable to discuss these points in some order of succession, 

 that we may harmoniously present the facts to those who may be in- 

 terested in them." In the present paper, I shall confine myself to the 

 families of the JJnionidoi, and Strepomatidoi^ which have a wider or 

 more general distribution than the other fresh-water groups. Be- 

 ginning our consideration of this question with the New England 

 States, we find no representative of the StreiJomatidce^ and ver^^ few of 

 the Unionidoe. Such of the latter as we do find, are of the types 

 that occur in a multitude of varietal forms, along the Atlantic slope, 

 east of the Appalachians, with a few having a wide westward and 

 southward range. But among these shells occurs one remarkable 

 anomaly of distribution in the presence of the Margaritana margariti- 

 fera^ Lam., an European species which occurs in the New England 



