46 WATER REPTILES OF THE PAST AND PRESENT 



The periods of the Paleozoic era are the Cambrian, Ordovician, 

 Silurian, Devonian, Carboniferous, and Permian, in the order as 

 given; those of the Mesozoic era are the Triassic, Jurassic, and 

 Cretaceous; those of the Cenozoic era, the Eocene, Oligocene, 

 Miocene, Pliocene, Pleistocene, and Recent. As a relic of an old 

 classification we still often divide the Cenozoic into two quite 

 arbitrary divisions, the Tertiary and the Quaternary, the latter 

 including the Pleistocene and Recent only. The same may be 

 said regarding the limits of each of these periods as of the eras; 

 the sole problem is to make each period contemporaneous through- 

 out the world, an exceedingly difficult problem, because no faunas 

 or floras have ever been the same over the whole earth. Indeed, 

 with the exception of some of the lowliest and most generalized 

 forms, or man himself, no species are the same throughout the 

 earth today. Inasmuch as we must depend upon the fossils in the 

 rocks for the determination of the ages, where none is quite the 

 same in strata of remote localities the identification becomes very 

 difiicult or even impossible. Nor are the periods, as accepted, 

 of equal or even approximately equal duration; the Cretaceous 

 period, for instance, was longer than all the remainder of the 

 Mesozoic, longer perhaps than all the time which has elapsed since 

 its close. 



The earliest animals with a backbone, or rather the earliest that 

 we call vertebrates — for some vertebrates have no vertebrae — 

 began their existence, so far as we know, in late Ordovician times, 

 as attested by fish bones in Ordovician rocks of Colorado and Utah. 

 The first evidences of the existence of air-breathing vertebrates 

 in geological history are footprints preserved in the uppermost 

 Devonian rocks of Pennsylvania. We call them amphibian because 

 they resemble footprints associated with amphibian skeletons in 

 later formations, and because the foot itself is still the most impor- 

 tant difference we know between fishes and the higher animals. 



In the rocks of the next great time division, the Mississippian, as 

 we call it in America, corresponding more or less closely with the 

 Lower or Subcarboniferous of other parts of the world, numerous 

 footprints of amphibians have been discovered, but no fossil 

 remains except a few from near its close in Scotland. From the 



