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, Halitheriwm 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 Paloplotheriwm, an Eocene genus of Tapi- 
ride or Paleotheride. 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, Acerotherium. The Suidz (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, Paleocherus, Cheromorus, 
all of a small or moderate size ; Hyopotamus, nearly as large as a 
tapir ; and Anthracotherium, 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 Anoplotheriade, 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 Synaphodus, known only from its 
teeth, which differ somewhat from those of the Anoplotheriwm 
which appears earlier in the Eocene formation. Another extinct 
family, Amphimericide or Xiphodontide, is represented by two 
