THE MIGRATIONS OF MARINE ANIMALS 285 



the surface or in the depths of its waters. A great 

 number of Ammonites possess an almost universal 

 geographical distribution, certain species being 

 found with identical or almost identical characteris- 

 tics from Central Europe to as far away as South 

 America, Natal, and Japan. Notwithstanding these 

 unfavourable conditions, it has been possible to ob- 

 serve, even among the Cephalopods, certain inter- 

 esting facts of migrations : the genus Virgatites, 

 so characteristic of the deposits of the upper Jurassic 

 of Russia (the Arctic province) spread in a short 

 period through Germany to the Boulonnais and to 

 Specton Cliff on the English North Sea Coast. To 

 explain this migration it seems difficult to discard 

 the hypothesis of a cold current starting from the 

 region of the White Sea and propagating itself along 

 a northern continent formed by Lapland, Finland, 

 and the Scandinavian peninsula. But an explana- 

 tion so simple does not appear sufficient to account 

 for other migrations in mass of certain groups of 

 Ammonoids which we find on several occasions invad- 

 ing European seas in Primary and Secondary times. 

 These intermittent invasions, which bring into the 

 regions of Central Europe certain genera of Cephalo- 

 pods till then unknown which have no origin in 

 earlier formations, seem especially connected with the 

 epochs of great incursions of the sea during which it 

 must have overflowed its earlier shores, and have 

 spread far over the solid continents, bringing with 

 it colonists from the ocean depths, or, at all 

 events, from more distant marine provinces. I will 

 quote, for example, the sudden introduction of the 

 group of Clymenias with the incursion of the 



