MAMMALIA 295 



ineptus) has been exterminated by man within the period of 

 two centuries ; and in the islands of Bourbon and Rodriguez 

 the " solitaire" {Pczopliaps) has also become extinct. Both 

 these birds had wings too short for flight. 



Class IV.— MAMMALIA. 



{Warm-blooded, Air-breathing, Viviparous Vertebrates.') 



Every calcified part of an animal, whether coral, shell, crust, 

 tooth, or bone, can preserve its form and structure when buried 

 in the earth during the changes there gradually operated in it, 

 until every original particle may have been removed and 

 replaced by some other mineral substance previously dissolved 

 in the water percolating the bed containing the fossil. A bone, 

 or other part so altered, is said to be " petrified." Not only are 

 all its outward characters preserved, but even the minutest 

 structure may be, and in most cases is, demonstrable in the 

 fine sections under the microscope. 



Fossil bones and teeth have been discovered in every 

 intermediate stage of alteration, from their recent state to that 

 of complete petrifaction. Eecent bones consist of a soft — com- 

 monly called animal or organic — basis, hardened by earthy 

 salts, chiefly phosphate of lime.* Fishes have the smallest 

 proportion, birds the largest proportion, of the earthy matter 

 in their bones. The soft part is chiefly a gelatinous substance. 



Proportions of Hard and Soft Matter in the Bones of the 

 Vertebrate Animals. 



FISHES. 



Salmon. Carp. Cod. 



Soft 60-62 40-40 34-30 



Hard 39-38 5960 65-70 



100-00 100-00 100-00 



* That this combination of phosphorus and calcium has ever taken place in 

 nature, save under the influences of a living organism, remains to be proved. 



