Rhinoceros, Ceratorhinus, and Atelodus. As there are so few existing species the subdivision of animals which agree in so many and such highly-characteristic features seems to be an unnecessary procedure. The existing Rhinoceroses are but a fragment of the total number of known forms from past epochs. The family is very markedly on the wane.
The genus Rhinoceros is characterised by its heavy build and thick, almost smooth, skin—smooth, that is to say, so far as concerns the slight development of hair—which is often thrown into folds. There is one or there are two horns on the fore-part of the head, which are, as has already been pointed out, structures sui generis, and not exactly comparable with the horns of other living Ungulates. There are three nearly equal toes on both fore- and hind-limbs. The canine teeth of existing species have disappeared; the incisors are, or are not, present; the molars and premolars are three and four in each half of each jaw.
The visceral anatomy of the Rhinoceros has been much investigated so far as concerns the Asiatic forms. A curious feature, which serves to discriminate some of the Asiatic species from others, is to be seen in the small intestine. In Rh. indicus[1] this gut is furnished with numerous long cylindrical narrow outgrowths "like tags of worsted"; in the allied Rh. sondaicus these tags are present, but are flatter and broader; while in the two-horned Rh. sumatrensis there are no tags at all, but only smooth valve-like folds. Another mark by which these species can be distinguished depends upon the variation in the presence or absence of certain glands imbedded in the integument of the foot—the so-called "hoof glands." These occur in Rh. indicus and Rh. sondaicus, but are absent in Rh. sumatrensis.
Sir W. Flower[2] studied some years since the skull features which serve to differentiate the existing forms.
In Rh. sumatrensis the two long downward processes of the squamosal bone, termed respectively post-glenoid and post-tympanic, do not unite below the auditory meatus. In this the species in question agrees with the African forms but not with the one-horned Asiatic species, where the two processes completely fuse. Again, another character, though perhaps less important,
