222 THE CHAIN OF LIFE. 



remarkable illustration of the persistence of similar groups of 

 creatures in successive ages in the same region, along with 

 diminution in magnitude and number of species toward the 

 modern times. 



Fig. 178, — Lower Jaw of Megatherium, dreatly reduced. Post-Pliocene of South, 



America. — After Owen. 



The Whale-tribe (Cetaceans) at once in the earliest Eocene 

 takes the place of the great Sea-lizards of the Cretaceous ; and 

 the oldest of the whales are in their dentition more perfect 



Fig. 179. — Ungual Phalanx and Claw-core of Megatherium. Greatly reduced. 



than any of their successors, since their teeth are each implanted 

 by two roots, and have serrated crowns, like those of the Seals. 

 The great Eocene whales of the South Atlantic {Zeuglodori) 

 (Fig. 180), which, have these characters, attained the length of 



