Bony dermal plates or scutes are less common among reptiles, though by no means rare. The turtles, as is well known, are almost completely inclosed in such an exoskeleton, bones which have coalesced more or less to form a box or carapace within which the head and limbs may be withdrawn for protection. In the modern crocodilians also the body is more or less protected by small bone plates forming rows on the back and sometimes on the under side. The ancient phytosaurs had similar plates. Not a few of the dinosaurs were more or less covered with bony scutes and sometimes with large bony plates or spines. Some modern lizards have bony plates over the body instead of horny scales. CHAPTER IV THE AGE OF REPTILES Geologists divide the history of the earth, since life first appeared upon it, into four general eras, the Proterozoic, Paleozoic, Mesozoic, and Cenozoic, that is, into eras of first life, ancient life, middle life, and recent life. These divisions were made long ago by geologists when it was believed that extraordinary changes, great cataclysmic revolutions, marked their limits. With a fuller knowledge of the life of the past we know that evolution has been continuous and uninterrupted; possibly accelerated or retarded at times, but without break. Were the earth’s history to be written anew, with our present knowledge, and with an unbiased mind, it is very doubtful whether many of the time divisions would have the same limits that they have now—whether the Paleozoic would terminate with the Carboniferous, or the Permian, or the Trias, or whether indeed we should think it necessary to make any primary divisions whatsoever. In other words, our greater knowledge of living and extinct organisms, and of the rocks which contain fossils, has made the problems of classification much more complex than they seemed to be formerly. It is much easier to classify organisms or rocks, or anything else, when we know only a few isolated kinds—much easier to draw divisional lines. Geological history is like a volume in which pages, leaves, and even whole chapters either are missing or are printed in languages which we understand only imperfectly. Where the lost or unknown parts belong, the largest divisions may be made, and possibly such may have been epochs of unusual activity, of diastrophic changes which greatly accelerated organic evolution. No one can say just where the dividing line should be drawn between the rocks of Paleozoic and Mesozoic age, or between the Mesozoic and Cenozoic, for there is none; the most that we can hope for is to make the divisions everywhere in the world conform to those first made for local reasons.