The geographical distribution of animals, with a study of the relations ...

—13G DISTRIBUTION OF EXTINCT ANIMALS. [part ii.Utah and Wyoming, we get a step further back, severalspecieshaving been discovered about the size of a fox with four toes infront and three behind. These form the genus Orohippus, andare the oldest ancestral horse known. Prof. Marsh points out theremarkably perfect series of forms in America, which, beginningwith this minute ancient type, is gradually modified by gainingincreased size, increased speed by concentration of the limb-bones,elongation of the head and neck, the canine teeth decreased insize, the molars becoming longer and being coated with cementtill we at last come to animals hardly distinguishable, specifically,from the living horse.Allied to these, are a series of forms showing a transition to thetapirs, and to the Palwotherium of the European Eocene. In thePliocene we have Parahippus ; in the Miocene Lophiodon, foundin the same formation and in the Eocene of Europe, and alliedto the tapir ; and in the Eocene, Palwosyops, as large as a rhinoceros,which had large canines and was allied to the tapir andPalccotherium ; Limnohyus, forming the type of a family Limnohyidse,which included the last genus and some others mentionedfurther on ; and Hyrachyus, allied to Lophiodon, and to Hyracodonan extinct form of rhinoceros. Besides these we have Lophiotherium(also from the Eocene of Europe) ; Diplacodon allied toLimnohyus, hut with affinities to modern Perissodactyla and nearlyas large as a rhinoceros ; and Colonoceras, also belonging to theLimnohyidse, an animal which was the size of a sheep, and haddivergent protuberances or horns on its nose. A remarkablegenus, JBathmodon, lately described by Professor Cope, and ofwhich five species have been found in the Eocene of New Mexicoand Wyoming, is believed to form the type of a new family,having some affinity to Palwosyops and to the extinct Brontotheridse.It had large canine tusks but no horns.The Bhinocerotidae are represented in America by the genusRhinoceros in the Pliocene and Miocene, and by Aceratheriumand Hyracodon in the Miocene. Both the latter were hornless,and Hyracodon was allied to the Eocene Hyrachyus, one of theLophiodontidse. In the Eocene and Miocene deposits of Utah,and Oregon, several remarkable extinct rhinoceroses have been