Among a number of peoples, some of whom form part of the Sioux, including the Minnitarees, Crows, Choctas, Creeks, Cherokees and Pawnees, cross-cousins are classed with parents and children exactly as in the Banks Islands, and exactly as in those islands, it is the son of the father’s sister who is classed with the father, and the children of the mother’s brother who are classed with sons or daughters. Further, among the Pawnees the wife of the mother’s brother is classed with the wife, a feature also associated with the peculiar nomenclature for cross-cousins in the Banks Islands. The agreement is so close as to make it highly probable that the American features of relationship have been derived from a social institution of the same kind as that to which the Melanesian features are due, and that it was once the custom of these American peoples to marry the wife of the mother’s brother. Here, as in the case of the cross-cousin marriage itself, the case rests entirely upon the terminology of relationship, but we cannot ignore the association in neighbouring parts of North America of features of relationship which would be the natural consequence of two forms of marriage which are known to be associated together elsewhere. I am indebted to Miss Freire-Marreco for the information that the Tewa of Hano, a Pueblo tribe, call the father’s sister’s son _tada_, a term otherwise used for the father, thus suggesting that they also may once have practised marriage with the wife of the mother’s brother. The use of this term, however, is only one example of a practice whereby all the males of the father’s clan are called _tada_, irrespective of age and generation. The common nomenclature for the father and the father’s sister’s son among the Tewa thus differs in character from the apparently similar nomenclature of the Banks Islands and cannot have been determined directly, perhaps not even remotely, by marriage with the wife of the mother’s brother. This raises the question whether the nomenclature of the Sioux has not arisen out of a practice similar to that of the Tewa. The terms for other relatives recorded by Morgan show some evidence of the widely generalised use of the Tewa, but such a use cannot account for the classing of the wife of the mother’s brother with the wife which occurs among the Pawnees. Nevertheless, the Tewa practice should keep us alive to the possibility that the Sioux nomenclature may depend on some social condition different from that which has been effective in the Banks Islands in spite of the close resemblance between the two.