chap, vi.] MAMMALIA OF THE OLD WORLD. 119 



Cetacea (whales). — These occur frequently in the Miocene de- 

 posits, four living, and five extinct genera having been described ; 

 but these marine forms are not of much importance for our 

 purpose. 



Sirenia (sea-cows). — These are represented by two extinct 

 genera, Halitheriwn and Trachytherium. Several species of the 

 former have been discovered, but the latter has occurred in 

 France only, and its affinities are doubtful. 



Ungulata. — Horses are represented by Hipparion and Anchi- 

 therium, the latter occurring in both Upper and Lower Miocene 

 and Eocene ; while Hipparion, which is more nearly allied to 

 living horses, first appears in the Upper Miocene and continues 

 in the Pliocene. 



Hippotherium, in the Upper Miocene of the Vienna basin, 

 forms a transition to Paloplotherium, an Eocene genus of Tapi- 

 ridse or Palseotheridse. Tapirs, allied to living forms, occur in 

 both Upper and Lower Miocene. Rhinoceroses are still found 

 in the Upper Miocene, and here first appear the four-toed horn- 

 less rhinoceros, Acerothermm. The Suida? (swine) are rather 

 numerous. Sus (wild boar) continued as far back as the Upper 

 Miocene ; but now there first appear a number of extinct forms 

 which have been named Hyotherium, Palceochcerus, Chceromorus, 

 all of a small or moderate size ; Hyopotamus, nearly as large as a 

 tapir ; and Anthracothcrium, nearly the size of a hippopotamus 

 and, according to Dr. Leidy, the type of a distinct family. 

 Listriodon, from the Upper Miocene of the Vienna basin, is 

 sometimes classed with the tapirs. 



We now come to a well-marked new family of Artiodactyle 

 or even-toed Ungulata, the Anoplotlicriidm, which consisted of 

 more slender long-tailed animals, allied to the swine but with 

 indications of a transition towards the camels. The only genera 

 that appear in the Miocene formation are, Chalicotherium, nearly 

 as large as a rhinoceros, of which three species have been found 

 in Germany and France ; and Syno.pilwdus, known only from its 

 teeth, which differ somewhat from those of the Anoplothcrium 

 which appears earlier in the Eocene formation. Another extinct 

 family, Amphimericidce or Xiphodontidcc, is represented by two 



