36 ANIMALS OF THE PAST 
soft or moist earth into which wind-driven 
dust or sand might lodge, or sand or mud be 
swept by the mimic flood caused by a thunder 
shower. 
So there are tracks in strata of every age; 
at first those of invertebrates: after the worm 
burrows the curious complicated trails of ani- 
mals believed to be akin to the king crab; 
broad, ribbed, ribbon-like paths ascribed to 
trilobites ; then faint scratches of insects, and 
the shallow, palmed prints of salamanders, and 
the occasional slender sprawl of a lizard ; then 
footprints, big and little, of the horde of Di- 
nosaurs and, finally, miles above the Cambri- 
an, marks of mammals. Sometimes, like the 
tracks of salamanders and reptiles in the car- 
boniferous rocks of Pennsylvania and Kansas, 
- these are all we have to tell of the existence 
of air-breathing animals. Again, as with the 
iguanodon, the foot to fit the track may be 
found in the same layer of rock, but this is not 
often the case. 
Although footprints in the rocks must often 
have been seen, they seem to have attracted lit- 
tle or no notice from scientific men until about 
