Allied to _Toxodon_ is the genus _Nesodon_. It was so named from an "island lobe" on the inner side of the upper molars. This creature, smaller than _Toxodon_, also differs from it in the fact that the dentition is complete, and in the pattern of the molars, which is rather more complex. There is still the slight projection upon the premaxillary bones, but the nostril is directed more forwards than in _Toxodon_. The zygoma, too, is massive. The second pair of incisors in the upper jaw and the outer (third) pair in the lower jaw form biggish tusks in the adult. These and the molar teeth are, however, finally rooted, and do not grow, as in _Toxodon_, from persistent pulps. The genus is from the older Tertiary of Patagonia. Five or six species have been described. Some are as large as a Rhinoceros, others as small as a Sheep. {216} 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.