126 FIELD GEOLOGY. 



existences will naturally vary according to the class of 

 animals by which they were originated, and setting 

 plants aside for the present, it will be worth while 

 reviewing the animal kingdom for the purpose of seeing 

 what groups are likely to be met with at all in the fossil 

 state, and which of them so occur in greatest frequency 

 and abundance. 



EEVIEW OF ANIMAL KINGDOM. 



VERTEBRATA. 



Mammalia occur rarely, except in recent fluviatile 

 deposits, and then generally in the shape of 

 separate bones and teeth. 



Birds from their aerial existence, are still rarer as 

 fossils, bones seldom occurring ; but their foot- 

 prints and even impressions of their feathers 

 are known. 



Reptilia. Eemains of all recent and extinct orders, 

 except the Ophidia and Lacertilia, are tolerably 

 abundant ; their bones, teeth, scutes, and in some 

 cases their eggs and coprolites being found. 



Amphibia are represented in certain formations by 

 their bones and teeth, as well as their tracks or 

 footprints. 



Pisces. The bones, teeth, and scales of fishes are com- 

 mon in almost every formation, from the Silurian 

 upwards. 



