The Sparassodonta are an interesting group of extinct mammals found in the Tertiary formations of Patagonia. They appear to have taken the place of true Carnivora in South America during most of the Tertiary period, as the Carnivorous Marsupials do in the modern fauna of Australia. Their relationship has been in dispute since they were first made known to science by the distinguished South American paæontologist Florentino Ameghino. In dental formula and other characters of the teeth and jaws they agree with the Carnivorous Marsupials, and show a high degree of adaptive specialization for predaceous habits. Dr. Ameghino was able to determine, however, from specimens in his collection, that the replacement of the premolars, in certain genera at least, was more complete than in modern marsupials, and approached that of the placental carnivora. He regarded them, therefore, as an intermediate group, ancestral to the Creodonta and modern Carnivora. Mr. Tomes, in his interesting study of thb enamel structure of the Creodonta and allied groups, has recently shown that in one genus of Sparassodonts the enamel shows the more highly differentiated structure of the true Carnivora, instead of the more primitive structure of Marsupial Carnivores. The collections made by the Princeton and American Museums in the Santa Cruz formation of Patagonia contain a splendid series of Sparassodont remains, including skulls and skeletons of the principal genera, which have recently been exhaustively studied by Dr. Sinclair. These show that the Sparassodonts agree with Marsupials, and especially with Thylacinus in almost all the distinctive features of dentition, skull, and skeleton characters by which they are separated from the placental mammals.