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F the cold-blooded, air-breathing animals known as true reptiles there are now in existence upon the globe more than four thousand species, classified by naturalists in four very distinct groups or orders—the Rhynchocephalia, Crocodilia, Chelonia and Squamata. Of the Rhynchocephalia, there is but a single species now living, the Tuatera or Sphenodon, confined to the islands off the northeast of New Zealand, and very rare. It is a small animal, seldom attaining a length of twenty inches, lizard-like in habits and appearance. Though now so near extinction, the order during Paleozoic times was an important one, both in size and variety. No older reptiles are known.

The Crocodilia, inclusive of the alligators and gavials (better garials) number at the present time scarcely twenty-five species, confined to tropical and subtropical shores of both the old and the new worlds. Of great size, cruel and sluggish in disposition, they seem to be a reminder of those times when size, rapacity and cruelty characterized their class on land, in the air and in the water. Though approaching extinction, these modern representatives of what was at one time a far more numerous and widely distributed order are the specialized descendants of an ancestral line scarcely less ancient than the rhynchocephalian. In form and size, and probably also in habits, the crocodilia have varied comparatively little throughout the greater part of their long period of existence upon the earth.

Were none of the Chelonians—the tortoises and turtles—now living, the extinct forms would appear to us among the most remarkable of vertebrate animals, so little interest do familiar things incite. Like the crocodilia and rhynchocephalia, they are among the oldest of reptiles; yet in number and variety at the present time they are inferior only to the lizards and snakes, and may truly be said, after the lapse of many millions of years, to be only now at or but recently past the zenith of their development. In the past there have been species perhaps twice or thrice as large as any now in existence—from the Bad Lands of Dakota there is known one sea-turtle twelve or more feet in expanse of shell, with a skull nearly as large as that of a horse—but in type of structure the very oldest that we know differed but slightly from some that are now living.