THE MIGRATIONS OP MARINE ANIMALS 291 



Two incursions of the sea characterize the Tertiary 

 geological history of this region : one corresponds 

 to the lower and middle Miocene (Burdigalian and 

 Helvetian stages), the other to the early Pliocene 

 (Plaisantian and Astian stages) ; between these two 

 marine phases there occurs an important phase of 

 retreat of the sea during the upper Miocene or 

 Ponticate stage. It results from this fact that there 

 exists no relation of direct descent between the 

 Miocene and Pliocene forms of the same kind of 

 Molluscs in this valley. The Pecten restitutensis 

 of the lower Miocene, for example, though near 

 akin to the Pecten latissimus of the Pliocene, always 

 preserves its distinctive characteristics, and no 

 transitional form can be detected between the two 

 species. But if we go to the basin of the Danube, 

 we note in the middle Miocene round Vienna, the 

 co-existence of the two forms : the first in the lime, 

 the second in the sand deposits. It is probably 

 in this eastern basin and under the influence of the 

 varied conditions of the environment that the 

 differentiation of the two species must have taken 

 place, one of which, the P. restitutensis, became 

 extinct without leaving descendants, while the 

 other, P. latissimus, spread by migration over the 

 whole of the Pliocene Mediterranean. 



This way of looking on the succession in time of 

 kindred forms could be supported by many other 

 examples. I shall also quote, with Fontannes, the 

 faunas of land Molluscs, Limnaeas, Planorbes, Hydro- 

 bias, and Valvata, so common in fresh water strata, 

 which mark the two phases of retreat at the end 

 of the Miocene and Pliocene in the Rhodanian Gulf. 



