Vol. 52.] STEATA BETWEEN THE KIMEEIDGIAN AND APTIAN. 545 



Petchorian Series, that is, the zones of Ammonites stenomphalus and 

 of Ammonites Keyserlingi, as being the Lower Neocomian of Poreal 

 type, notwithstanding the affinity of their fauna with that of the 

 underlying zones of the Aquilonian, of which the Jurassic age is 

 thus more strongly defined. 



But if it should prove locally inconvenient to fix the lower limit 

 of the Cretaceous at a lower level than the base of the zone of 

 Hoplites neocomiensis and regalis, then these Petchorian beds and 

 their equivalents may be considered as an intermediate series 

 between the two systems, to be placed either with the Jurassic or 

 with the Cretaceous, after the stratigraphical and faunistic rela- 

 tions of the beds with Ammonites Marcousanus and Gevrilianus of 

 Central Europe have been thoroughly studied, and their equivalents 

 and relationship to the underlying and overlying beds more com- 

 pletely understood. This, however, seems to me a less convenient 

 course than that which I have proposed above, since it still leaves the 

 limits of the systems indefinite, though no doubt it might in some 

 countries suit better the local conditions as displayed in the litho- 

 logical or palaeontological sequence. 



In the Syzran region the next underlying horizons to the beds 

 above described are a thin band (20 centimetres) of bituminous 

 shale and a thin band of greenish sand, both very poor in fossils ; and 

 still lower come a sandstone and conglomerate with Aucella volgensis, 

 trigonoides, Fischeri, Ammonites kashpuricus, subclypeiformis, and 

 many other ammonites allied to nodiger and kashpuricus. 



In the government of Riasan the succession of the Upper Jurassic 

 and Neocomian beds is as follows : — The beds with Cardioceras alter- 

 nans are overlain by a thin band of glauconitic sand and phosphatic 

 nodules containing several species of Virgatites. This bed thins out 

 here and there, and is reduced to a narrow band of phosphatic 

 nodules only (as at Speeton). The next bed is a glauconitic sand, 

 with Ammonites fragilis and catenulatus and phosphatic sandstone 

 with A. kashpuricus, subclypeiformis, Behmnites mosquensis, many 

 species of Aucella (mosquensis, Fischeri), etc. This bed is very 

 thin, and passes insensibly upwards into the zone of Hoplites riasan- 

 ensis, without changing its lithological character. 1 This passage 

 is indeed indicated only by the substitution of the forms of the group 

 of Hoplites riasanensis for Ammonites kashpuricus and subclypei- 

 formis, the rest of the fauna remaining almost unchanged. The 

 same Belemnites and Aucellce form a characteristic feature in the 

 fauna and impress upon it a boreal character, which contrasts 

 markedly with the meridional type of certain of the Hoplites more 

 or less resembling Upper Tithonic forms. This occurrence of Upper 

 Tithonic forms at the top of the zone of Craspedites kashpuricus 

 and Oxynoticeras subclypeiforme, constituting the uppermost zone 



1 These relations are to be seen at the village of Kusminskoie. There are 

 other places (Old Riasan) where the nodiger-zone, and even the catenulatus- 

 zone, is absent, and the bed with the Huplites riasa?iensi.<i-group is much thicker 

 than at Kusminskoie. Possibly such cases indicate that the riasane/isis-t'nunn 

 appeared there somewhat earlier. 



