328 LLOYD'S NATURAL HISTORY. 



teeth, whose compressed crowns had the edges surmounted by 

 well-marked serrations ; and, what is more remarkable, their 

 bodies appear to have been invested with a bony armour com- 

 parable to that of Crocodiles. 



The last of the Tertiary Mammals that we have to notice are 

 Opossums (Didelphys), remains of which have been detected 

 both in the Hord\vell beds and in the Lower Eocene sand of 

 Kyson. It is almost superfluous to add that Opossums, which 

 in Oligocene and Eocene times were widely spread over 

 Europe, are now confined to America, where they attain their 

 greatest development in that half of the continent lying to the 

 south of the isthmus of Darien. The relegation of the origin- 

 ally European genus to the New World is somewhat analagous 

 to the banishment of the nearest living allies of the British 

 Jurassic Mammals to Australia, and is a well-marked instance 

 of that gradual disappearance of the lower types of Mammalian 

 life from the western regions of the Old World, with the 

 development of higher forms, which seems to have been such a 

 characteristic feature in the evolution of the present Faunas 

 of the globe. 



