MANUAL OF NATURAL HISTORY. 577 



thousand fossil species of Bivalves have likewise 

 been discovered in these strata, including the genera 

 Ostrea, Pecten, Venericardia, Cytkerea, Lucina, 

 Corbida, Tellina, Nucula, Crassatella, &c. The 

 fossil oysters often occur in extensive beds, as in 

 the tertiary clays near Woolwich, and in the lower- 

 most sands and clays of the London basin. 



II. — Miocene Group. 



The Miocene forms the middle group of tertiary 

 strata ; it is composed of fresh-water and estuary 

 beds of marls, imperfect limestones, and clays, 



The lower part of this formation in England, con- 

 sisting of a mass of calcareous marls, shells, and 

 small corals, is called the " Coralline Crag of Suffolk;" 

 five hundred species of mollusks have been obtained 

 from it. Out of England, the faluns of Touraine, the 

 beds of Bordeaux, the conglomerate of Piedmont, and 

 part of the molasse of Switzerland, belong to this 

 group. 



In the Falunian stage, the Metaxytkerium, a cu- 

 rious extinct mammal, allied to the Lamantin and 

 Dugong, has been found. In beds of sand and marl of 

 this group, we see at Darmstadt in Germany thebones 

 of one of the most gigantic mammals yet discovered, 

 the Dinotherium, an animal as large as an elephant, 

 with tusks in the lower jaw curved downwards ; it 

 is allied to the Mastodon and Tapir, and is supposed 

 to have hung by its tusks to the banks of rivers and 

 lakes, like the Walrus to the floating masses of ice 

 in polar seas. Belonging to the Miocene stage are 



