MAMMALS—URSIDAE—URSUS. Dik 
The skulls of all the Carnivora vary exceedingly with different age, the shape of the indi- 
vidual bones, and the general proportions of the head changing in a striking degree. This is 
particularly the case in the bears, so much so thet any reliable comparisons between different 
species must be based on examinations of specimens of the same age as indicated by the temporal 
fosseel, and the sagittal crest, as well as by the condition of the teeth. This last, however, is 
an uncertain basis of comparison, as the amount of attrition of the crowns of the molars and 
incisors depends much more upon the general nature of the food than the age. Bears in 
districts where animal sustenance and soft vegetables abound retain the integrity of the teeth 
much more than those which feed upon acorns, nuts, or other seeds requiring considerable 
grinding. 
Another source of difficulty in the determination of species of the bears is owing to the fact 
that the size at maturity varies considerably in different individuals of the same species. The 
sexes, too, are said to differ constantly. Thus, according to Blainville, the males of both 
Ursus americanus and U. arctos differ from the females in having the forehead flatter and less 
arched transversely than in the female and young male. 
The teeth, too, may vary considerably in size in two specimens of one species, where the 
other proportions are the same. Thus, in two skulls of Ursus americanus, from one locality in 
Louisiana, of the same size and age, the posterior upper molar of one (1154) measures 0.92 
inches ; that of the other, (9889 ,) 1.10 inches. I may here mention that in No. 1154 there is 
one upper premolar additional to the usual number on one side, two being developed between 
the first and third. There are thus five premolars and two molars. 
In describing the skulls of the bears it will be better to select as the type one of middle age, 
and afterwards indicate the variations in the very old and very young ones. 
The number of species of bears belonging to North America is somewhat uncertain. Those 
usually assigned are the black bear, Ursus americanus—the grizzly, U. horribilis—and the polar 
or white sea bear, U. maritimus. The cinnamon bear of the Rocky Mountains has usually been 
considered a variety of the black bear. Skulls of a small brown bear from the Copper Mines of 
New Mexico, probably the cinnamon bear, show conclusively a difference of species, and the grizzly 
bear of the same neighborhood exhibits peculiarities not found in the grizzly of the Pacific 
coast. There may thus be five species, and to this we must probably add the Barren Ground 
bear of Richardson, referred by him to Ursus arctos of Europe. This may, however, prove to 
be the same with the small grizzly of the Rocky Mountains; in which event it must be consid- 
ered as entirely distinct from U. arctos. 
For the sake of illustrating the comparative sizes of the skull in different species of North 
American bears, with the relative proportions of each, I have prepared the following table, 
based upon selections from the large stock of specimens before me. 
28 L 
