The first striking feature is the dental formula. As formerly given, it is inaccurate, there being two great tushes on either side of the upper jaw, instead of one, as described. At first sight, I thought it might be a meristic variation, but both of my skulls show the same arrangement on both sides, and these are the first two skulls which have been found complete to the front end, and neither is by any means a young individual. The dental formula would then read 2 0 3 3 -------. 1 0 2 3 Upper incisor 1 is a rootless, permanently growing tush about a fourth smaller than inc. 2, but of the same character, being oval in cross section and having enamel on the front face only. These first incisors are directed downward, so that their ends stand between and very slightly in front of the second incisors. The end of each is worn bluntly round in contrast to the beveled end of inc. 2. The second incisor is larger, rootless, and permanently growing, with a hollow base, enamel on the front face only, and oval in cross section. Both these teeth have a layer of cement on them, extending some distance beyond the alveolus. The tips are worn in a long bevel on the posterior side, very much as is the case on the incisors of rodents. The third upper incisor, the canine, and premolar 1 are lacking, a long diastema occupying their place, out of which they have been crowded by the development of the enormous root of inc. 2, which extends 150 mm. and more back into the jaw. _P. romeri_ is distinguished from the others by pm. 1 being present. The teeth of the upper premolar-molar series have their crowns expanded, and the two series of either side have moved toward each other; until in front they are in contact while in the rear they are only 50 mm. apart, narrowing the palate in a unique manner, and giving the impression that the palate is mostly a grinding surface. The premolars are completely molariform and the whole series is at an advanced stage of specialization. Premolar 2 is three-rooted, with a triangular crown on which are three mamma-like tubercles, the larger one in front, and two behind. As the crown is worn, these unite into a flat, grinding surface, surrounding a central pit which opens behind. Premolars 3 and 4 and all the molars are large, four-rooted, quadrilateral teeth, each with two transverse crests running clear across the crown, and with a small cingulum across the anterior margin. Before they are worn, the top of each crest is tuberculated, and the cingulum is crenulated. In wearing, the anterior face of each crest is ground down; so that instead of the crown wearing to a level surface, it retains throughout life two oblique grinding surfaces.