Typotherium.—Typotherium (Fig. 192), also called Mesotherium, from the same locality as Toxodon, was an animal rather larger than a Capybara, and of much the same general appearance. Its skeleton is completely known, and shows a singular combination of characters, resembling Toxodon or a generalised Ungulate on the one hand, and the Rodents, especially the Leporidæ, on the other. In the presence of clavicles it differs from all known Ungulates, and in having two pairs of lower incisors from all Rodents. The teeth are i ¹⁄₂, c ⁰⁄₀, p ²⁄₁, m ³⁄₃ = 24.

From the Tertiaries of various parts of South America a number of forms more or less closely allied to Toxodon and Typotherium have been recently described, but as many of them are very imperfectly known, and there is much doubt as to their generic position, it will be unnecessary to refer to them further.

It will thus be seen that, although our knowledge of many of these forms is still very limited, we may trace among them a curious chain of affinities, which would seem to unite the Ungulates on the one hand with the Rodents on the other; but further materials are required before we can establish with certainty so important a relationship, one which, if true, would alter materially some of the prevailing views upon the classification of mammals.

Group Tillodontia.

Here may be noticed a remarkable group of animals, called by Marsh, Tillodontia, the remains of which are found abundantly in the Lower and Middle Eocene beds of North America. They seem to combine the characters of the Ungulata, Rodentia, and Carnivora. In the genus Tillotherium of Marsh (probably identical with the previously described Anchippodus of Leidy) the skull (Fig. 193) resembled that of the Bears, but the molar teeth were of the Ungulate type, while the large incisors were very similar to those of the Rodents. The dental formula is i ²⁄₂, c ¹⁄₁, p ³⁄₄, m ³⁄₃. The first pair of incisors was very small; the upper molars were tritubercular, while the lower ones had crescentoid ridges as in Palæotherium. The skeleton resembled that of the Carnivores, but the scaphoid and lunar bones were distinct, and there was a third trochanter on the femur. The feet were plantigrade, and each had five digits, all with long pointed claws. In the allied genus Stylinodon all the teeth were rootless. Some forms were as large as a Tapir.

These, with other more or less closely allied animals, such as Calamodon and Psittacotherium, constituting a group called Tæniodonta, are included by Cope in his large order Bunotheria, to which also the existing Insectivora are referred. The dentition of some of these forms makes a remarkable approximation towards a Rodent type, while it has been suggested that there are also signs of remote Edentate affinities. The constantly increasing knowledge of these annectant forms adds to the difficulty so often referred to in this work of establishing anything like a definite classification of the heterodont mammals. An incisor tooth from the Swiss Eocene has recently been referred to Calamodon.