There is no doubt about the close alliance of the two genera just referred to. It is more doubtful whether Homalodontotherium and its allies should be placed, as they often are, in the neighbourhood of the Toxodonts. Homalodontotherium owes its name to its even row of teeth without a diastema. It was a creature of equally large size with Toxodon, and also came from the Tertiary strata of Patagonia. The teeth are the typical forty-four, and the molars like those of a Rhinoceros; they are, however, brachyodont and not hypselodont as in Toxodon. This genus, however, shows an important difference from the Rhinocerotidae and from the other Toxodontia in the fact that it was five-toed, and that the bones of the carpus and tarsus are set in relation to each other in the linear serial fashion.
Undoubtedly a near relative of Homalodontotherium is Astrapotherium. This creature was of equal bulk, and was also Patagonian in range. The teeth are reduced in number, but the animal was provided, like a Wild Boar, with great tusks, which were, however, formed by the incisors. This animal is very imperfectly known; it is the form of the molars and the large size of the incisors which have led to its association with the Toxodontia. As to the resemblance of the teeth of this genus and of Homalodontotherium to those of Rhinoceros, it is difficult to regard it as evidence of near affinity. The likeness is probably to be looked upon as a case of parallelism in development. Exactly the same explanation is possibly to be given to the likeness which the teeth of Toxodon and Nesodon show to Rodents, or even to Edentates. As to their affinities Zittel observes:—
"The entirety of their osteological characters argues for the Toxodon a separate position in the neighbourhood of the Perissodactyla, Proboscidea, Typotheria, and Hyracoidea. The relations to the Rodentia rest mainly upon the converging development of the teeth, not upon true relationship."
Sub-Order 6. PROBOSCIDEA.
Large vegetable-feeding animals, usually scantily covered with hair, and with the nostrils and upper lip drawn out into a long proboscis. Digits five on both limbs. Femur and humerus not bent upon lower leg and fore-arm in a position of rest. Skull
