DUAL NOMENCLATURE. 157 



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 repre- 

 sent 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, ^o the name of the era, thus : the Eocambrian, or the period of 

 the Olenellus fauna, the Mesocambrian, or the period of the Para- 

 doxides fauna, the Neocambrian, or the period of the Dikelloceph- 

 alus fauna, and in the Devonian, the Eodevonia^i, Mesodevonian, 

 and Neodevoiuan, 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 HemercB,^ it is 

 doubtful whether our knowledge of the history of organisms is 

 sufificiently advanced to enable paleontologists to define the fauna 

 of an epoch so that it can be made use of for more than a thou- 

 sand miles of geographical extent, and the hemercB 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 knowl- 

 edge we have of the time relations of the contained fossils. In the 

 case in hand, the fauna of the Catskill is not sufificiently 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 represen- 

 tative 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 



' S. S. BucKMAN : The Bajocian of the Sherborne Districts, its Relation to Subjacent 

 and Superjacent Strata. Q. J. G. S., Vol. XLIX., p. 479-522, Nov., 1893. 



