152 PALEONTOLOGY 



ICHNOLOGY* 



In entering upon the genetic history of the class of 

 reptiles, we have to inquire, as in that of fishes, in what period 

 of the earth's history the class was introduced, and under what 

 forms ; at what period it attained its plenary development, in 

 regard to the size, grade of structure, number and diversities 

 of its representatives ; and the relations which the existing 

 members of the class bear to its past condition. Fifteen years 

 ago, the oldest known reptilian remains were those of the so- 

 called " Thuringian Monitor," from the Permian copper-slates 

 of Germany. Five years ago, the batrachian Apatcon, or Ar- 

 ehegomurus was discovered in a Bavarian coal-field ; and about 

 the same period, footprints in carboniferous sandstones of 

 North America, had been recognized as evidence of the com- 

 mencement of reptilian existence at that period of the earth's 

 history. Air-breathing ambulatory animals may leave other 

 evidence of their former presence upon earth than their fossil- 

 ized remains. 



There are several circumstances under which impressions 

 made on a part of the earth's surface, soft enough to admit 

 them, may be preserved after the impressing body has perished. 

 When a shell sinks into sand or mud, which in course of time 

 becomes hardened into stone, and when the shell is removed 

 by any solvent that may have filtered through the matrix, its 

 place may become occupied by crystalline or other mineral 

 matter, and the evidence of the shell be thus preserved by a 

 cast, for which the cavity made by the shell has served as a 

 mould. If the shell has sunk with its animal within it, the 

 ic matrix may enter the dwelling-chamber as far as the 

 retracted soft parts will permit ; and as these slowly melt away, 

 their place may become occupied by .crystallized deposits of 



* Ix''°*» a footstep, and a^os 



