On the Grouping of some Divisions of so-called ‘Jurassic’ Time

I Introduction . A considerable portion of this paper was written before the appearance of a communication on Dundry Hill. A reference to the ideas set forth in this paper will be found in that communication, in Table IV, facing p. 696. [When this paper was first presented to the Society the Author had designated the Ages by the terms Bathonian, Bajocian, etc., proposing, in effect, to continue the usage carried out in the communication above mentioned, to employ these terms for chronological purposes only. They have been used formerly, as for instance by Renevier, for both stratigraphical and chronological divisions:— Bathonian Stage or Bathonian Age. However, it was pointed out to the Author that this plan would lead to confusion; and he then proposed to use, as distinctly chronological terms for Ages, names taken from dominant ammonite-genera. There is an advantage in this plan, seeing that the chronological divisions are based entirely upon zoological phenomena. And there was an admitted disadvantage in retaining for chronological purposes names such as Bathonian, Bajocian, etc., taken from places where particular strata were developed. But the change has a wider effect if a congruous series of terms is to be produced. Jurassic is equally open to the same objection as Bathonian, and it will require to be replaced by a term taken from some zoological phenomenon. The divisions of Jurassic—Eo- and Neojurassic—fall into the same category; they may be easily replaced by such terms as Arietidan and Stepheoceratidan Epochs, which appropriately mark the changes of