While resembling the parabola in extending to infinity, the curve has closest affinities to the ellipse.
One of the two living species was, indeed, described so long ago as the year 1863, under the preoccupied name of Hyracodon, but attracted little or no attention, as its affinities were not fully recognized.
Here may be noticed three genera of large extinct marsupials from the Pleistocene of Australia whose affinities appear to ally them to the wombat-group on the one hand and to the phalangers on the other.
The affinities of the creature are clearly with the phalangers.
As regards the affinities of the creatures to which these jaws belonged, Professor Osborn has referred the Triconodontidae and Amphitheriidae, together with the Curtodontidae (as represented by the English Purbeck Curtodon), to a primitive group of marsupials, while he has assigned the Amblotheriidae and Stylacodontidae to an ancestral assemblage of Insectivora.