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 batraehian Apateon, or Ar chegoeaurue 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 plastic 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 any silicious, calcareous, or other crystallizable matter that may have been held in solution by water percolating the matrix, and such crystalline deposit may receive and retain some colour from the soft parts of which it thus becomes a cast.
Evidences of soft-bodied animals, such as Actinice and Medusce, and of the excremental droppings of higher animals, have been thus preserved. Fossil remains, as they are called,
of soft plants, such as sea-weeds, reeds, calamites, and the like, are usually casts in matrix made naturally after the plant itself has wholly perished.
Even where the impressing force or body has been removed directly or shortly after it has made the pressure, evidence of it may be preserved. A superficial film of clay, tenacious enough to resist the escape of a bubble of gas, may retain, when petrified, the circular trace left by the collapse of the burst vesicle. The lightning flash records its course by the vitrified tube it may have constructed out of the sandy par ticles melted in its swift passage through the earth. The hailstone, the ripple wave, the rain-drop, even the wind that bore it along and drove it slanting on the sand, have been registered in casts of the cavities which they originally made on the soft sea-beach ; and the evidence of these and other meteoric actions, as sun-cracks and frost-marks, so written on imperishable stone, have come down to us from times incal culably remote. Every form of animal life that, writhing, crawling, walking, running, hopping, or leaping, could leave a track, depression, or foot print, behind it, might thereby leave similar lasting evidence of its existence, and also to some extent of its nature.
The interpretation of such evidences of ancient life has much exercised the sagacity of naturalists since Dr. Duncan, in 1828, first inferred the existence of tortoises at the period of the deposition of certain sandstones in Dumfriesshire, from the impressions left on those sandstones, and the casts after wards formed in those impressions. The faculty of interpreting has been still more racked by similar evidences of more extra ordinary footprints, probably of large batrachian reptiles, first noticed in 1834 at Hildberghausen in Saxony, in sandstones of the same geological age as those in Scotland.