100 URSID.E. 



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 speleeus, 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 speleeus 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, 



