286 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



upper Devonian time, that of the Psilocerata with 

 the incursion of the infra-Liassic, the invasion of 

 the Amalthece and of the Ccelocerata with the deepen- 

 ing of the middle Liassic seas, the arrival of the 

 Oppeliidae and of the Haploceratidse with the incur- 

 sion of the Bajocian era, that of the Cardiocerata 

 with the great Callovo-Oxfordian incursion, the 

 sudden expansion of the Hoplites and of the 

 Holcostephani with the incursion of the upper 

 Tithonian era, the apparition of the Desmoscerata 

 and the Mortonicerata with the Valanginian, of the 

 Holcodisci with the Hauterivian, of the Silesitce, and 

 the Costidisci in the Barremian, of the Douvelleicerata 

 in the Aptian, of the Scaphitce, and the StoliczkaicB 

 in the Cenomanian, and lastly, that of the Pseudo- 

 ceratitidae with the Turonian. The phenomena of 

 incursion, alternating with the epochs of the re- 

 treat of foreshores or regressions, seem thus to have 

 been one of the most important causes of the re- 

 peated renewal, or of the intermittent remodelling 

 of the faunas of animals of the high seas. 



The same causes of migration have naturally 

 reacted, with even greater intensity, on littoral 

 faunas, coast-animals being still more sensitive 

 than the high-sea types to the various changes 

 which affect the marine environment. Migra- 

 tions determined by a modification of the tempera- 

 ture of the waters of the sea seem above all to depend 

 on the direction of the currents, some warm and 

 superficial, others cold and deep. P. Fischer and 

 after him Locard have shown that the existing 

 littoral Molluscs in the arctic regions of the North 

 Atlantic have propagated themselves towards the 



