THE EOCENE PERIOD. 309 



Pouched Quadrupeds (Marsupials), almost the lowest group 

 of the whole class of the Mammalia. In the Eocene rocks, 

 on the other hand, numerous remains of Quadrupeds have 

 been brought to light, representing most of the great Mam- 

 malian orders now in existence upon the earth, and in many 

 cases indicating animals of very considerable dimensions. We 

 are, in fact, in a position to assert that the majority of the 

 great groups of Quadrupeds with which we are familar at the 

 present day were already in existence in the Eocene period, 

 and that their ancient root-stocks were even in this early time 

 separated by most of the fundamental differences of structure 

 which distinguish their living representatives. At the same 

 time, there are some amongst the Eocene quadrupeds which 

 have a " generalized " character, and which may be regarded 

 as structural types standing midway between groups now 

 sharply separated from one another. 



The order of the Marsupials including the existing Kan- 

 garoos, Wombats, Opossums, Phalangers, &c. is poorly 

 represented in deposits of Eocene age. The most celebrated 

 example of this group is the Didelphys gypsorum of the 

 Gypseous beds of Montmartre, near Paris, an Opossum very 

 nearly allied to the living Opossums of North and South 

 America. 



No member of the Edentates (Sloths, Ant-eaters, and Arma- 

 dillos) has hitherto been detected in any Eocene deposit. 

 The aquatic order of the Sirenians (Dugongs and Manatees), 

 with their fish-like bodies and tails, paddle-shaped fore- 

 limbs, and wholly deficient hind-limbs, are represented in 

 strata of this age by remains of the ancient " Sea-Cows, " to 

 which the name of Halitherium has been applied. Nearly 

 allied to the preceding is the likewise aquatic order of the 

 Whales and Dolphins (Cetaceans), in which the body is also 

 fish-like, the hind-limbs are wanting, the fore-limbs are con- 

 verted into powerful " flippers " or swimming-paddles, and 

 the terminal extremity of the body is furnished with a 

 horizontal tail-fin. Many existing Cetaceans (such as the 

 Whalebone Whales) have no true teeth; but others (Dol- 

 phins, Porpoises, Sperm Whales) possess simple conical teeth. 

 In strata of Eocene age, however, we find a singular group 

 of Whales constituting the genus Zeuglodon (fig. 228), in 

 which the teeth differed from those of all existing forms in 

 being of two kinds, the front ones being conical incisors, 

 whilst the back teeth or molars have serrated triangular 



