332 PALEONTOLOGY. 



ineptus) has been exterminated by man within the period of 

 two centuries ; and in the islands of Bourbon and Eodriguez 

 the " solitaire" (PezopJiaps) 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, when every original particle may have been 

 removed and replaced by some other mineral substance pre- 

 viously 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 pre- 

 served, but even the minutest structure may be, arid in most 

 cases is, demonstrable in the fine sections under the micro- 

 scope. 



Fossil bones and teeth have been discovered in every in- 

 termediate stage of alteration, from their recent state to that 

 of complete petrifaction. Becent bones consist of a gelatinous 

 basis hardened by earthy salts, chiefly phosphate of lime * 

 Fishes have the smallest proportion, birds the largest propor- 

 tion, of the earthy matter in their bones. 



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 59*60 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. 



