Yol. 52.] STRATA BETWEEN THE KIMERIDGIAN AND APTIAN. 551 



III. General Considerations. 



Let us now, by the light of these results, glance at the course of 

 geological events in our region during the final stages of the Jurassic 

 epoch and the beginning of the Cretaceous. 



In Kimeridgian time over Eastern and partly over Central Russia 

 there existed an extensive sea, communicating with the "West 

 European Kimeridgian sea and stretching to the east and north- 

 east far beyond the boundaries of Europe. 1 In this basin were 

 developed certain ammonites, appearing in the West European sea 

 as 4 cryptogenic ' types. 2 The fauna of the Russian Kimeridgian sea, 

 though possessing a general resemblance to the "West European 

 fauna, exhibits many peculiarities of its own, as, for instance, the 

 abundance of the belemnites of the Magnifici and Eacplanati-growps, 

 the abundance of Cardioceras (C. subtilicostatum, C. Volga?) and the 

 presence, though rare, of Aucella. 



In Centra] Russia, at the end of the Kimeridgian age, the sea 

 disappeared, and there occurred a certain amount of destruction of 

 the Kimeridgian beds, 3 but in Eastern Russia a further evolution and 

 migration of faunas took place, and the ammonites of the Bleicheri- 

 group came into existence (derived probably from the Kimeridgian 

 Perisphinctes), and Belemnites magnificus, Troslayanus, and explanatus 

 appeared as the descendants of Belemnites Oweni, Panderi, and brevi- 

 aocis of the previous period. In "Western Europe the fauna of this 

 sea, separated for a time by a tract of land, evolved some peculiar 

 characteristics, and there appeared certain cryptogenic ammonites in 

 it, such as Ammonites portlandicus (gigas). With the beginning of 

 the Virgatites epoch, a new ' hydrocratic ' displacement of the shore- 

 lines brought the sea again into Central Russia and even farther into 

 Poland, and there was a communication between this Russian sea 

 and that of the West European Bononian, and apparently also that 

 of England. In this manner we seek to explain the presence of 

 Virgatites in the Portlandian of Boulogne, the presence of Aucella 

 Pallasi in the Lower Portlandian (Upper Kimeridgian of English 

 authors) shales of Spilsby, and the presence of the fragments of 

 Virgatites and Belemnites, cf. absolutus in the Lower Coprolite-bed 

 of Speeton. 



The physical conditions of the region remained without great 

 change in the succeeding age of Ammonites giganteus, but the 

 Amelia, seem to be absent or very rare at this time in Western 

 Europe, probably owing to some conditions unfavourable to their 

 existence or to the presence of some inimical forms of animal life in 

 this region. 



In the northern part of England an interruption of sedimentation 



1 Witness the Aucella in the Kimeridge Clay of the Volga region and of 

 the Petchora region, and certain faunistic affinities between the Kimeridgian 

 Hoplites of Eastern Russia and some of the ammonites of Tibet. 



2 Neumayr, ' Ueber unvermittelt auftretende Cephalopodentypen im Jura 

 Mittel-Europa's,' Jahrb. d. k.-k. geol. Reichsanst. vol. xxviii. (1878) p. 37. 



3 Hence the occasional discovery of Kimeridgian Hoplites in the Portlandian 

 phosphatic nodules and in the Boulder Clay. 



Q. J. G. S. No. 207. 2 p 



