Attention is drawn to anomalies in the nomenclature of the niobatetantalates, and the fergusonite series as used by Dana is renamed the fergusonite-formanite series. Chemical variations in this series are discussed with particular reference to H₂O, Ca, Ti, Yt, individual lanthanons, Th, and U. Members of the series usually show heavy lanthanons exceeding light lanthanons but one specimen shows high Gd+Tb+Dy.