by William Swainson

“ Nevertheless it is observed, by those who study natural affinities, that when the links between two distinct forms of animals are so complete, that their dissimilarities are lost in intermediate and undefinable gradations, it is then that the natural arrangement is most likely to be discovered. ”

“ In our former Series, we have frequently mentioned the Fluviatile Bivalve Shells, of which the genus Unio appears to be the type. In a group which present so few certain characters, either for discriminating the species or characterizing the genera, we have, with other writers, held different opinions at different times. ”