This surprising diversity of groups, each so widely isolated, as is implied by separation as subclasses—divisions of almost the highest rank—shows that the class developed in favorable circumstances that stimulated enterprise, so to speak, and resulted in rapid variation of habits, terrestrial, aquatic, arboreal, and aerial, and consequently of adaptive structure. The fact that most of the subclasses are extinct also shows us that the story of the Reptilia is mainly a tale of the departed glory recorded in the archives of the rocks; and we shall hardly be able to understand living reptiles properly without knowing something of their prehistoric development into the dominance to which they rose in the Mesozoic era, which we call Age of Reptiles, and their subsequent decadence.
The first subclass covers certain most ancient skeletons and parts of skeletons that naturalists are not yet agreed are true reptiles, some considering them stegocephalian amphibians. Anyway, they indicate plainly that it was from that group of Amphibia that the variety sprang that developed into what, in time, became the distinct reptilian type. The first distinct product of this departure from the stegocephalian stock appears in the fossils of a division of the second subclass, the Prosauria (pro, "before"; saurus, a "lizard"), named Rhynchocephalia ("beakheads"), which, although lizardlike in general form, retain many amphibian characteristics of structure. Now the amazing and extremely interesting thing about this is that a representative of this earliest of true reptiles is still living—probably the premier peer among all vertebrates, reckoned by length of ancestry. This most primitive of reptiles, illustrating how hundreds of ancient species known to us only by a few bones must have appeared and acted in life, is the tuatara of New Zealand, catalogued in science as Sphenodon punctatum.
It has the shape and general appearance of a big lizard, dull in color and with a granulated rather than scaly hide, and an oddly shaped head, toothless in the adult, when the jaws become somewhat like a horny beak. Yet it is not a lizard any more than it is a crocodile or a turtle, but combines features of all three in its anatomy. Hence it is what naturalists term a synthetic or generalized race (as is the case with all very primitive creatures) out of which more and more specialized groups and species may be, and are, developed, each sorting out and strengthening some particular characteristic of structure, continuously modified by adaptations to habits and environment until a separate type results. The ribs, for example, in the tuatara are remarkable for the presence of hooklike processes that project backward from each rib over the next rib behind it; such processes occur elsewhere only in the crocodiles and the birds. Behind the breastbone are rodlike bones embedded in the muscles of the belly; they occur again in the ancient fish lizards and modern crocodiles, and probably gave rise to the under shield of the turtles. And so on.
The tuatara is verging on extinction. It has nearly disappeared from the mainland of New Zealand, but is now protected on some small adjacent islands where it dwells in burrows which it digs and then shares with petrels. During the greater part of the day the tuataras sleep; and are fond of lying in the water, being able to remain submerged for hours without breathing. They feed only upon other animals.
The third subclass (Theromorpha, "beast-shaped") comprises very ancient reptiles whose remains lie in the rocks of Permian and Triassic age, principally in South Africa, and exhibit a skull, and especially teeth, so much resembling those of carnivorous mammals (for instance, those of a dog) that at first their true nature was mistaken. These creatures have excited the most profound interest, not only because they present so many differences from the Prosauria, but also, and chiefly, because it is from their ranks that we are able to trace, with no small degree of certainty, the origin of the Mammalia.
THE SOURCES OF TORTOISE SHELL AND TERRAPIN STEW
The turtles and tortoises are of a very ancient group (Chelonia) and one very distinct among reptiles, by reason of their armor. What is known as tortoise shell is the series of horny plates, in some species of beautiful texture, in others thin and dull, or even leathery in character, that covers the underlying bones that form the real protection to the animal's body. In embryo (unhatched) turtles the skeleton is much like the ordinary four-footed type, with the vertebræ separate, a full series of ribs, and the limb bones in their proper places. As growth proceeds, however, changes occur rapidly, but least in the oceanic "leathery" turtle, in whose skin nodules of bone expand and join into a mosaic of plates covered with a thick, coriaceous hide. But this skin remains quite separate from the skeleton beneath, which fact places this animal in an order Athecæ ("lacking a case"), quite by itself. All other chelonians are classified in a second order Thecophora ("case-bearing"), and in them the changes that go on in the skin to produce the turtle's shell are far more complete.
If you peel off the horny shields on the upper shell, or "carapace," you will find beneath them a central, lengthwise row of squarish plates of bone, on each side of these a row of similar plates, and outside of these a marginal row of small plates—all knit together at the edges, the zigzag lines of juncture, or "sutures," being plainly visible.
When we dissect a turtle we find no layer of skin or flesh beneath these plates, but discover that they lie directly on the bones of the skeleton and are a part of it. This is what has happened: The vertebræ have grown together, and the backbone is a tube upon which the original nodules in the skin have become fixed, and have broadened into the central line of plates. Those nodules that lay above the ribs have become fused with them so that no trace of ribs is left, except where their heads have become fused with the backbone, and they have broadened into the side rows of plates; and the marginal skin has become transformed into the marginal plates. Similar alterations have produced the under shell, or "plastron," replacing the skin; and adaptive changes have altered the usual relations of the limb bones to the rest of the external skeleton. The carapace and plastron are usually connected by a "bridge" of bone.