The Rhinoceroses of America were mostly hornless. _Diceratherium_ is an exception; but in many cases it had two parallel not successive horns, and these were, to judge from the slight prominences, but feeble in development, and perhaps hardly exactly comparable with the formidable weapons of the Old-World forms. _Aceratherium tridactylum_, with indications of paired horns, may be ancestral to _Diceratherium_. The American forms have weak and slender nasals in correspondence with the absence of horns; the sagittal crest is retained in contradistinction to the great flattened surface of the skull in the horned Rhinoceroses. _Aceratherium_ of both divisions of the globe probably represents the ancestral group of the horned and the hornless forms. This being the case it is highly interesting to note a distinct convergence in the quite {260} separate American genera towards the European horned genera. A genus sometimes united with _Aceratherium_, but still differing from it in some points, is _Aphelops_ (_Teleoceras_).[171] This animal is more nearly approximated to "the modern standard" of Rhinoceroses than is its possible ancestor _Aceratherium_. The skeleton in general is more robust, even surpassing that of modern forms, and approaching the _Hippopotamus_. There is a reduction in the upper incisors, which are limited to two pairs, and the lower molars {262} are reduced to five. The lower incisors are only two. The sagittal crest is less marked; the fifth digit is reduced to a tiny nodule representing the metacarpus. It had a small nasal horn. There are numerous other details of likeness to modern Rhinoceroses in this creature, which has only community of descent with them from the older hornless forms, such as _Aceratherium_ and _Caenopus_. In the genus _Peraceras_ the upper incisors are as completely gone as in the living African Rhinoceroses. [Illustration] FIG. 135.--Skeleton of _Aphelops (Teleoceras) fossiger_. x 1/15. (After Osborn.) The most ancient rhinocerotine types[172] are the Hyracodonts and the Amynodonts. They both date from the Eocene, and became extinct in the succeeding Oligocene. _Hyracodon_[173] (Fig. 134) was "an agile, light-chested, and rather long-necked" type, resembling a Horse in build. There were no horns present, but the hoofs were more like those of the Horses than of the existing Rhinoceroses. These animals were apparently plain dwellers and defenceless, which is held to account for their compact hoofs and outward similarity to a Horse. The genus is Oligocene. The dental formula is I 3/3 C 1/1 Pm 4/3 M 3/3.