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nian, Carboniferous (constituting the eras of Paleozoic time), Triassic, Jurassic and Cretaceous (the eras of Mesozoic time), and Eocene, Neocene, and Recent (the eras of Cenozoic time). Each of these eras is known, not by the formations which represent it, but by the fossils which characterize it. Proceeding in the same manner, I would propose that we subdivide the eras into Periods, according to the lines which paleontologists have generally come to recognize as convenient and natural, making a three-fold (or in some cases a two-fold division) of each era, and that they be distinguished by prefixing the syllables Eo, Meso and Neo, to the name of the era, thus: the Eocambrian, or the period of the Olenellus fauna, the Mesocambrian, or the period of the Paradoxides fauna, the Neocambrian, or the period of the Dikellocephalus fauna, and in the Devonian, the Eodevonian, Mesodevonian, and Neodevonian, and in the same manner for each of the other eras.

Although attempts have been made to make finer divisions of the time-scale, into Epochs and, lately, into Hemeræ,^[1] it is doubtful whether our knowledge of the history of organisms is sufficiently advanced to enable paleontologists to define the fauna of an epoch so that it can be made use of for more than a thousand miles of geographical extent, and the hemeræ of single species cannot be considered as precisely alike in two distinct geological provinces.

In the application of this time-scale to any particular case the degree of minuteness of definition will depend upon the knowledge we have of the time relations of the contained fossils. In the case in hand, the fauna of the Catskill is not sufficiently definitive in itself to enable us to be more precise than to assign it to the Devonian Era. This is affirmed by the fact that, in England, the old Red Sandstone is known not as a part of, but as the representative of the whole Devonian. If we attempt to define the age of the Catskill formation by faunas immediately preceding or succeeding it, the definition in time is equally indistinct, because