The genus Notharctus was founded by Leidy in 1870 upon a small fossil jaw which had been found in the Eocene formation near Fort Bridger, Wyoming. Leidy was not able to refer the animal to any existing order of mammals. He noted its resemblances to carnivorous mammals on the one hand and to certain supposed Eocene pachyderms on the other.
Marsh, who described a similar jaw fragment the next year, 1871, also noted its resemblances to the supposed suilline pachyderm Hyopsodus. In October, 1872, however, Marsh described some better specimens, which included parts of the limb bones. He remarked that the principal parts of the skeleton of these animals were formed much as in some of the Lemurs, especially the limb bones, and he referred the animals to . . .