Just how to classify these three series of rhinoceroses and rhinoceros-like animals, so as most accurately to express their mutual relationships, is a question that has received several answers. One method suggested is to include them all in a single family and to make a subfamily for each of the three well-distinguished series; this is the arrangement which personally I should prefer. A second plan is to accord family rank to each of the three groups; while the most elaborate scheme, that of Professor Osborn, is as follows: for the rhinoceroses, in the broader sense, he makes two families, the Rhinocerotidæ and the †Hyracodontidæ, and divides the former into four subfamilies, which include all of the true rhinoceroses, living and extinct, of the Old and New Worlds, and the latter into two subfamilies, the †Hyracodontinæ and †Amynodontinæ. It is not a matter of very great moment as to which of these three schemes is followed, and I shall therefore adopt the one proposed by Professor Osborn, in order to avoid, so far as possible, the confusing effect of different methods of classification. [Illustration: FIG. 180.—†Cursorial rhinoceros (_†Hyracodon nebrascensis_), White River stage. Restored from a skeleton in the Museum of Princeton University.] As before mentioned, the subfamily of the †hyracodonts (†Hyracodontinæ) became extinct in White River times, during most of which it was represented by the single genus _†Hyracodon_, whence are derived the names for the family and subfamily. The series was purely North American, and no member of it has ever been found in any other continent. The species of _†Hyracodon_ were altogether different in appearance and proportions from the true rhinoceroses, being lightly built, slender, cursorial creatures, suggestive rather of horses than of rhinoceroses, to which they bore much the same relation as the slender-limbed, narrow-footed †lophiodonts did to the tapirs (_see_ p. 326); in size, they were somewhat taller and considerably heavier than a sheep.