(8) In the phyla composed of massive animals the principle of change agreed with that exemplified by the †titanotheres, increasing body-weight being the determining factor in both cases. When this increase began to be decided, the reduction of digits ceased at the point which had already been reached in any particular series, three in both manus and pes in the true rhinoceroses, four in the manus and three in the pes in the †amynodonts. Very heavy animals require broad, columnar feet to support them, and hence the similarity of appearance in such widely separated groups as elephants, rhinoceroses and hippopotamuses, not to mention several extinct orders and families. Among the larger and heavier rhinoceroses, as in those of the present time, there was great variety in the proportionate lengths of the limbs, body and feet. In brief, the great complexity of the history of the rhinoceroses is due to the many divergent and parallel phyla into which these animals may be grouped. Broadly speaking, they may be subdivided into the slender, cursorial types and the heavy, slow-moving types, the former developing in a manner similar to that shown by the horses, while the latter were modified after the fashion of the †titanotheres. Obviously the load to be supported by the legs and feet was a very important factor in determining the character of evolutionary change. II. †ANCYLOPODA. †CLAWED PERISSODACTYLS The very extraordinary and aberrant animals which are referable to this suborder have been understood only since the year 1888, for, as was shown in an earlier chapter (p. 41) their scattered parts had been assigned to two different mammalian orders, the skull to the perissodactyls and the feet to the pangolins, or scaly anteaters (Pholidota) of the Old World, since it occurred to no one that the same animal could have such a skull and teeth in combination with such feet. The history of the Ancylopoda is still very incomplete, only four genera, of the lower Pliocene, middle and lower Miocene, and the middle Eocene respectively, being at all adequately known, but even in this imperfect form the story is worth telling. The suborder was probably of American origin and its most ancient known member existed in the middle Eocene. Both in Europe and North America the group persisted into the lower Pliocene and it is believed, though not clearly demonstrated, that in eastern Asia it continued even into the Pleistocene. All the genera of the suborder may be included in a single family. 7. _†Chalicotheriidæ. †Chalicotheres_