Vol. 52.] STRATA BETWEEN THE KIMERIDGIAN AND AFITAN. 543 



of the Aquilonian stage. Notwithstanding the presence of numerous 

 Cretaceous species iu these beds in Russia and in England, I found 

 it on the whole more convenient to class them as Jurassic, noting, 

 however, that they formed the uppermost Jurassic strata, lying at 

 the extreme limit of the two systems. In adopting this view I was 

 guided by the following considerations. Both in Russia and in 

 England the fauna of this horizon is more closely related to the 

 fauna of the underlying than to that of the overlying beds; and 

 also the undoubtedly Neocomian fossils, such as Ammonites Astieri, 

 Hoplites regalis, Belemnites pistillirostris, etc., are found only above 

 this horizon, their appearance bringing all at once a new element into 

 the fauna. Moreover, the Cretaceous species recognized among the 

 representatives of the genus Polyptychites in the Petchorian strata 

 had mainly been found in the Hils Beds of Germany, whose strati- 

 graphy has been incompletely studied. 



My scheme of the Russian subdivisions indicated that there was 

 a well-marked stratigraphical and faunistic break between the zone 

 of Polyptychites KeyserUngi and the Simbirskian Neocomian beds, 

 with Simbirskites versicolor, Decheni, and discofalcatus, which latter 

 represented apparently the boreal development of the Upper Neoco- 

 mian with possibly also the upper part of the Middle Neocomian ; 

 and that the beds with Hoplites regalis and Astieria Astieri were 

 absent in Russia. But there were still other horizons, not strictly 

 defined palaeontologically and stratigraphically, which by their 

 position, as it was then understood, might be regarded as lying 

 near the limit between the Jurassic and Cretaceous systems. The 

 fauna of one of these horizons displayed an affinity to the Upper 

 Tithonic fauna, and the fauna of another was closely related to 

 that of the zone of Polyptychites KeyserUngi. I refer here to the 

 zone of Hoplites riasanensis, and to that of Olcostephanus hoplitoides 

 of the government of Riasan. I had not then the necessary 

 material for denning strictly the position of these horizons in the 

 stratigraphical series, but I presumed that one of them, namely the 

 zone of Olcostephanus hoplitoides, might represent a Lower Neocomian 

 horizon of a still unknown boreal type, and that the other — that is, 

 the zone of Hoplites riasanensis — might represent a mixed type of 

 the Lower Neocomian. 



Shortly after the publication of these results it was demonstrated, 

 by my further researches and the palseontological studies of M. W. 

 Stchirowsky * in the Geological Museum of Moscow, that in the 

 northern part of the government of Simbirsk (districts of Alatyr 

 and of Kurmysh) there exists a horizon the fossils of which show 

 affinities both with the zone of Craspedites subditus and nodiger, 

 and with that of Polyptychites KeyserUngi. Having seen the great 

 interest which attached to the study of these horizons, I entered 

 upon new researches, in the zones both of Olcostephanus hoplitoides 

 and of Hoplites riasanensis, in the region of their greatest develop- 



1 ; Ueber Ammoniten der Genera Oxynoticeras und Hoplites aug dem nord- 

 fiimbirsk'schen Neocoua,' Bull. Soc. Imp. Nat. de Moscou, n. s. vol. vii. (1893) 

 p. 369. 



