224 



U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



polar* bear. The premolars between this and the canines are either very small or wanting 

 entirely in all the skulls of the grizzly, while in arcios there are two quite large ones. 



Corresponding differences exist in the lower molars, which are largest in the American bear, 

 though not in the same proportion as the upper. The posterior molar is much more elongated 

 in the grizzly, equalling the second in advance of it, while in U. arctos it is not more than 

 three-fourths this length. An important difference exists in the fourth molar from behind. 

 This in both exhibits one central and elevated compressed pointed lobe. In the grizzly, there 

 is a tubercle at the base of this lobe postero-internally, from which a low angular ridge passes 

 to the posterior end of the tooth, parallel with a similar ridge, passing back from the lobe 

 and a little exterior to the middle line of the tooth. These two ridges meet behind and enclose 

 a small valley. Now, in the U. arctos the inner tubercle with its ridge are wanting, and there 

 only is a central ridge along the posterior half of the tooth. In this feature of a single ridge 

 there is quite a close agreement with the Ursus americanus and cinnamoneus. The incisor teeth 

 seem to have much the same characters in both species. 



In view of the very great difference in the size of the teeth in the smallest grizzly bear and 

 the largest of our smaller species, it is hardly necessary to go into any detailed comparisons of 

 the skulls of the species. The most important characteristics, independent of size, are to be found 

 in the narrower head, straighter profile, and more flattened and concave forehead, greater pro 

 portional size of the posterior upper and lower molars, and numerous other features in the grizzly. 



The following table exhibits the detailed measurements and proportions of the skull in several 

 grizzlies, from various localities, as well as of a skull of the polar bear : 



Measurements. 



