It was suggested by the writer, in a paper read before the Geological Society in 1893,* that duality of nomenclature was desirable in order to discriminate between the divisions of the time scale and those of the formation scale, and later (1902–’3), when the revised rules of nomenclature and classification were being prepared by the United States Geological Survey,† it was still thought that the two scales might be discussed separately if only the criteria of discrimination and the nomenclature were kept strictly distinct. In 1903,‡ in a paper published in the Bulletin of this Society, the shifting of faunas during the continuance of their biological integrity was shown to be a fact, and it was pointed out that consequent precaution was necessary in using fossils with precision in classifying formations. In papers recently completed and now being published as Bulletin 244 . . .