MIDDLE MIOCENE PERIOD. 93 



sandstone has preserved to Switzerland the fauna of the shallow 

 coast-line ; and the varied mixture of the shells which lie to- 

 gether in all directions and are often broken and rolled/ and the 

 sharks' teeth and fragments of wood which lie among them, in- 

 dicate a shore-formation; while the animals of the subalpine 

 Molasse, which lie together often in great masses, with the valves 

 of the bivalves not unfrequently united, probably lived in the 

 sandy locality where they are found and where they had been 

 covered up. This would sufficiently explain why the species in 

 the Shell- sandstone and the subalpine Molasse are differently 

 associated, and would afford a reason for some of the common 

 species being more abundant in the subalpine Molasse, and for 

 others prevailing more in the Shell-sandstone. 



We therefore unite the animals of the Shell-sandstone and of 

 the subalpine Molasse into one stage, and compare them with 

 those of the other Miocene stages. 



Of the Mollusca of the Cretaceous sea, no single species re- 

 mained in the Miocene sea ; and even from the Eocene deposits 

 only five species (Solecurtus coarctatus, Corbulomya complanata, 

 Pholadomya arcuata, Tellina crassa, and Area nived) are found 

 in the Swiss Helvetian stage of the Miocene. Hence a great 

 change of forms had taken place since the preceding epochs. 

 With the Tongrian stage of the Miocene the Helvetian marine 

 Molasse shares only 15 species; whilst it has 118 species in 

 common with the Aquitanian, and 303 in common with the third 

 stage. It has almost an equal number of species (namely 299) 

 in common with the younger stages (the Tortonian, Placencian, 

 and Astian of Carl Mayer) and with the existing fauna. 



In about three fourths of its species the molluscan fauna of 

 the Swiss Helvetian stage agrees with the molluscan fauna which 

 inhabited the European seas in the third stage of the Molasse ; 

 it also agrees in about the same proportion with the molluscan 

 fauna of the younger Upper Miocene and the Pliocene formations; 

 and about one third of these species has descended to the present 

 fauna. A similar proportion may also be observed in other parts 

 of the sea which at that time covered Central Europe *. 



* Prof. Homes has described 476 marine univalves (or 500, including land 

 and freshwater shells) from the basin of Vienna. Of these, 99 species are cer- 

 tainly still living; 27 more are doubtful. Thus there are from 21 to 26-5 per 



