THEORY OF THE EARTH. 99 



speaking, those which rest immediately upon the 

 coarse limestone. It is chiefly these genera which 

 occupy the regular beds that have been deposited 

 from fresh-water, or certain alluvial beds of very 

 ancient formation, generally composed of sand and 

 rolled pebbles, and which were perhaps the ear- 

 liest alluvium of that ancient world. Along 

 with these there are also found some lost species 

 of known genera, but in small numbers, and some 

 oviparous quadrupeds and fishes, which appear to 

 have been all inhabitants of fresh-water. The beds 

 which contain them are always more or less covered 

 by alluvial beds, containing shells, and other ma- 

 rine productions. 



The most celebrated of the unknown species, 

 which belong to known genera, or to genera close- 

 ly allied to those which are known, such as the 

 fossil elephants, rhinoceroses, hippopotami, and 

 mastodons, do not occur along with those more 

 ancient genera. It is in the alluvial formations 

 alone that they are discovered, sometimes accom- 

 panied with marine shells, and sometimes with 

 fresh-water shells, but never in regular stony beds. 

 Every thing that is found along with these spe- 

 cies is either unknown like themselves, or at least 

 doubtful. 



Lastly, the bones of species which are appa- 

 rently the same as those that are still found alive, 

 are never discovered, except in the last alluvial 



