The _maxillary_ does not show the outward bulge at the third premolar, which is so marked in the fox. The alveolus is straighter, and the palatine plates are comparatively thicker and flatter. The infraorbital foramen is oval, and not so much compressed as in the fox, to which it corresponds very nearly in position, though situated slightly forward as in _Hyænodon_. _Measurements._ _Upper Jaw._ M. Length of third premolar ·007 Breadth of third premolar ·004 Length of fourth premolar ·007 Breadth of fourth premolar ·007 _Lower Molars_, from Dr. Leidy. Length of last premolar ·0075 Length of first molar ·009 These exhibit nearly the same proportionate size as in the gray fox. _Genus_ ----. _Species_ ----. _Sacrum_ (Plate IX., Fig. 8).--This peculiar sacrum is composed of only one true vertebra; there may have been one or more pseudo-sacrals, but this is not certain. The centrum is very long, strongly depressed, and straight on the inner margin, not curved as in the sacrum of most mammals. The anterior articular face is much depressed, and is one third larger than the posterior. The neural canal is low and subtriangular, resembling very much that of _Canis_. The pleuropophysial plates for articulation with the ilia are large and stout. The laminæ are heavy and concave on their upper side, supporting a very long, stout spine, which is retroverted and decidedly tuberous at the end. The pedicles are deeply notched behind; and on the fore part, just inside the metapophyses, there is a deep fossa. The chief features of this sacrum are decidedly carnivorous; but to what genus or family it should be referred we are unable to say. It has some of the characteristics of _Canis_, but the length and retroversion of the spine, as well as the size of the centrum, prevent this classification. In the general form of the pleuropophysial plates it approximates to the _seals_; while in its angle and curvature, it partakes of the character of the _Ursidæ_. The chief point of interest in this fossil centres in the fact that it was found only a few feet from the brain cast that is described below. _Measurements of Sacrum._ M. Length of centrum ·031 Long diameter of anterior articular face ·024 Long diameter of posterior articular face ·017 Width of neural canal ·019 Height of neural canal ·011 Length of neural spine ·036 Extreme width of sacrum ·052 MEGENCEPHALON. Megencephalon primævus. _Gen. et spec. nov._