DAKOTA AND NEBRASKA. 91 



17. The fragment of a skull about the same size as that of the preceding specimen, 

 but with the forehead and face above flatter than usual. Nasal bones flat, broad. 

 Posterior nasal triangle acute, twelve lines long and ten broad. Teeth small, but 

 slightly larger than in the preceding specimen. 



All the specimens above mentioned belonged to adult animals of various ages, as 

 indicated by the condition of the teeth. 



Three additional specimens, consisting of skulls more or less complete, not yet 

 arrived at maturity, hold nearly the same size, form and proportions as the specimen 

 first indicated and represented in figure 1, plate VI. All the specimens yet retain 

 the deciduous molar teeth, but the anterior two jsei'manent true molars also occupy 

 their functional position, and the last of the series was about to protrude. In all, the 

 sagittal crest is undivided to the anterior border of the parietal, which is more or less 

 deeply notched for the reception of the frontal summit. 



In one specimen the nasals are wide, and the posterior nasal triangle acute with a 

 length of twelve lines and a width of nine lines. In a second specimen the nasals 

 are less parallel at the sides than usual, and more sigmoid ; and the posterior nasal 

 triangle is acute, ten and a half lines long and ten wide. In the third specimen the 

 posterior nasal triangle is obtusely rounded, and is fourteen lines long and nine wide. 



The table on page 92 exhibits the comparative measurements of the seventeen 

 specimens above indicated. Many of the measurements are, however, only ap^Droxi- 

 mative or estimated, as the points of departure in the specimens are in frequent 

 instances broken. 



The teeth of Oreodon GuTbertsoni usually present uniformity of character in the 

 different fossils. In size they are generally proportionate with the size of the skulls, 

 the more robust of the latter usually presenting more robust teeth, with many of the 

 details of structure better developed. Thus the basal ridge becomes more e^'ident 

 and usually somewhat roughened, and other ridges of the premolars are more promi- 

 nent, while the intervals appear more depressed. In some instances, however, the 

 teeth appear to have been liable to variation in size and development not propor- 

 tionate with the size of the skull, and thus specimens exist in which large skulls 

 possess comparatively small teeth and the reverse. 



