CHANGES OF CLIMATE 315 



forms were extirpated from districts which they had 

 long inhabited, while in their place came migrations 

 of northern species. This modification made itself 

 felt both on terrestrial and marine life. Thus in 

 the Atlantic Ocean a number of northern shells, 

 which had pushed their way southward even as far 

 as the coasts of the Spanish peninsula, were able to 

 enter the Mediterranean when a connection was opened 

 between that inland sea and the main ocean outside. 

 It is interesting to note that among the shells intro- 

 duced into the Mediterranean basin at this time were 

 Astarte borealis, Buccinum groenlandicum, Cyprina island- 

 ica, Panopaea norvegica and others whose appellations 

 sufficiently indicate the latitudes where they now find 

 their chief home. On the land, too, such quadrupeds 

 as the reindeer and the now extinct mammoth wan- 

 dered from the plains of Lapland and Russia to the 

 shores of Italy. Eventually when the refrigeration 

 gave way to the return of more genial conditions, 

 the northern invaders died out. They have no living 

 descendants now in the south of Europe. 



The grey clay which forms the lower division of 

 the Roman Pliocene series is best seen on the right 

 side of the Tiber where it forms the lower half of the 

 ridge of the Vatican and Monte Mario, and where 

 for more than five-and-twenty centuries it has supplied 

 material for making the bricks of which ancient and 

 modern Rome has been so largely constructed. The 

 same clay has been found at lower levels on the 

 opposite side of the river. On the flanks of the 

 Pincian Hill, at the Piazza di Spagna, it was exposed 

 about twelve years ago in some excavations connected 



