442 MR. S. S. BTJCKMAIir ON THE GEOtTPING OF SOME [Aug. 1 898, 



31. On the Grouping of some Divisions of so-called 'Jurassic' 



Time. By S. S. Buckman, Esq., F.G.S. (Eead April 6th, 



1898.) 



Contents. 



Page 



Introduction 442 



I, General Considerations 443 



II. The Ages of the ' Jurassic ' Period 447 



III. The Divisions of a Portion of ' Jurassic ' Time, with Table I 451 



IV. A Genealogy of some ' Jurassic ' Ammonite-Genera, with Table II. 451 



V. Notes on certain Generic Names 452 



YI, List of Genera arranged under Families 459 



YII. Summary 462 



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 communi- 

 cation, 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 com- 

 munication above mentioned, to employ these terms for chronological 

 purposes only. They have been used formerly, as for instance by 

 Eenevier, 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 pheno- 

 mena. 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 i)e replaced by a term taken 

 from some zoological phenomenon. The divisions o£ 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 the ammonite- 

 fauna. Jurassic and T ri ass ic it is not so easy to replace; so, for 

 the present, they remain. They are strictly stratigraphical terms ; 

 but they are used in this paper with inverted commas for chrono- 

 logical purposes. Thus ' Jurassic ' Period denotes really the period 



1 ' Dundry Hill,' by S, S. Buckman & E. Wilson, Quart. Jouin. Geol. Soc. 

 vol. Hi (1896) pp. 669-720. 



