Ungulate Phenacodus (see p. 202). In the next stage (an embryo of 25 mm.) the humerus has slightly decreased in proportionate length, and has come to be more like that of Hipparion. In both of these embryos it should be noted that the ulna is complete and separate from the radius. In the second of the two it has more distinctly acquired the form which it will possess in the adult. The second metacarpal—one of the splint bones of the adult—is tipped with a small nodule of cartilage, which is clearly the representative of one or more of the phalanges belonging to that digit.
Fam. 2. Tapiridae.—The Tapirs may be distinguished from the Horse and from the Rhinoceros tribe by a few characters, which are as follows:—
The dentition is generally the full one of forty-four teeth. The premolars in the more ancient forms are unlike the molars, but like them in more recent forms. The molars of the upper jaw have two crests parallel and united by an outer crest. The fore-feet have four, the hind-feet three toes.
The family is fully as ancient as that of the Equidae, but the specialisation of the toes never advances so far. The modern representatives of the order are, so far as the feet are concerned, in the condition of very early representatives of the equine stock. Nor do the teeth of the Tapirs ever reach the complicated pattern of that presented by at least the modern Horses, or indeed of the Palaeotheres. Apart from this it is not an easy matter to distinguish accurately between these several families, including the Lophiodontidae, which, as already mentioned, is placed nearer to the Tapiridae than to the Palaeotheriidae. Indeed the differentiation of these two families, the Tapiridae and the Lophiodontidae, seems to be a matter of the greatest difficulty. The difficulty is well emphasised by the fact that naturalists disagree most profoundly as to the relations of various genera of extinct Tapir-like animals. For Mr. Lydekker the genus Lophiodon includes also the American genera Isectolophus and Systemodon, which are placed by Zittel in the sub-family Tapirinae as opposed to Lophiodontinae, which contains Lophiodon and Helaletes. The existing Tapirs can be differentiated from the existing Horses with great ease, as the following account of the existing genera will show.
The genus Tapirus is now met with only in South and
