692 MANUAL OF ZOOLOGY. 



as if it were composed of two separate teeth united on one 

 side by their crowns ; and it is this peculiarity which is ex- 

 pressed by the generic name. The species of Zeuglodon are 

 Eocene and Miocene. The species of Squalodon are Miocene 

 and Pliocene. 



As regards the distribution of the Cetacea in time, no mem- 

 ber of the order has yet been detected in any Secondary 

 deposit. The Zeuglodonts, as just remarked, extend from 

 the Eocene to the Pliocene, Zeuglodon itself being the oldest 

 Cetacean at present known. The Ziphioid Whales begin in 

 the Pliocene, as do the Catodontidce ; but the Delphinida are 

 known to occur in the Miocene. The Balcenidce are not 

 known to have existed earlier than the Pliocene. 



CHAPTER LXXII. 



UNGULATA. 



ORDER VI. UNGULATA. The order of the Ungulata, or 

 Hoofed Quadrupeds, is one of the largest and most important 

 of all the divisions of the Mammalia. It comprises three 

 entire old orders namely, the Pachydermata^ Solidungula, 

 and Ruminantia. 



The first of these old divisions that of the Pachydermata 

 included the Elephants, Rhinoceros, Hippopotamus, Tapirs, 

 and the Pigs, all characterised, as the name implies, by their 

 thick integuments. The name is still used to express this fact, 

 though the order is now abandoned, and is merged with that 

 of the Ungulata; the Elephants alone being removed to a 

 separate order under the name of Proboscidea. 



The second old order that of the Solidungula or Solipedes 

 included the Horse, Zebra, and Ass, all characterised by the 

 fact that the foot terminates in a single toe, encased in an 

 expanded hoof. The name Solidungula is still retained for 

 these animals, as a section of the Ungulala. 



The third old order that of the Ruminantia includes all 

 those animals, such as Oxen, Sheep, Goats, Camels, Giraffes, 

 Deer, and others, which chew the cud or "ruminate," and 

 have two functional toes to each foot, encased in hoofs. The 

 name Ruminantia is still retained for these animals, as consti- 

 tuting a most natural group of the Ungulata. 



All these various animals, then, are now grouped together 



