eohippus n. An extinct primitive dog-sized 4-toed Eocene mammal, the earliest horse known in the line of descent of the modern horse. It is classed in the extinct genus Hydracotherium. Called also dawn horse.
... back to five-toed creatures proved correct. One of the new links was indeed discovered before his lecture had passed through the press, and he was able to add in a footnote some details of the structure of the four-toed Eohippus from the lower Eocene beds. Further discoveries have connected these with the five-toed ancestors of the Tapirs, and there is the strongest reason to suppose that we now know as nearly as possible the line of ancestry of the horse back ... — Thomas Henry Huxley; A Sketch Of His Life And Work • P. Chalmers Mitchell
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