100 URSID A. 
more than doubt that the cold and bracing sea-breezes 
inhaled by the still flatter-headed Polar Bear, should be 
less efficient in expanding the sinuses along the respiratory 
tract, than the musty air of the sepulchral retreats in which 
the Cave Bears slept of old. 
Existing Bears, regarded as distinct species by modern 
zoologists, do in fact differ in the relative convexity of their 
forehead, and the flat-headed species, as the Polar and 
American Bears, are unquestionably not those which 
habitually respire the least pure and invigorating air. 
Instead, therefore, of speculating on the atmosphere as a 
physical cause of the inflation of the bony cells, it would be 
more profitable, if it were possible, to trace the relationship 
between the different degrees of development which the 
frontal sinuses may present in different species of Bears, and 
their peculiar habits and modes of life. We may thus, I 
think, see the reason why, in the piscivorous species of the 
Polar ice, the receptacles of air in the bones of the head 
are least developed, viz., to offer least resistance to its pro- 
gress through the water when diving after its prey. 
The opposite extreme in the condition of the frontal 
sinuses of the Ursus speleus, may have had some corres- 
ponding relation to the habits of that gigantic extinct 
species. 
From the great proportional size and more complicated 
tubercular surface of the posterior molar teeth, especially in 
the upper jaw, and from the greater complication on the 
crown of the smallest persistent molar in the lower jaw, 
one might be led to suppose that the Ursus speleus fed 
more on vegetables than the Grisly Bear does. In which 
case it might be inferred from the slight traces of abrasion 
in the teeth of full-grown ‘specimens, that the vegetable 
food, in whatever proportion it entered into their diet, 
