On the other hand, the Squamata (so called because of the scaly covering which characterizes them) are the most modern of all reptiles. They number now more than thirty-four hundred species, about equally divided between the lizards and snakes, the only modern representatives of the order. In the remote past so far as we can trace the broken line of descent, the ancestral forms had but a single conspicuous offset, the rapacious mosasaurs, which had, however, a comparatively brief existence. The lizards, like the snakes, probably go back little or no further than the Age of Mammals. They are, with few exceptions, terrestrial animals of small size and inoffensive habits; the largest living examples scarcely exceed six feet in length, though within the period of man's existence there have been lizards five or six times as long.
Snakes or serpents, the latest, and in many respects the most specialized, of all reptiles, have not yet reached the culmination of their development, a statement which perhaps may not be truthfully made of any other group of reptiles. Their geological history is insignificant and scanty. The venomous serpents especially present the latest modifications of reptilian structure, perhaps the very latest antecedent to the final extinction of the whole class.
No reptiles at the present time walk erect, as did many of the extinct kinds. Their progression is essentially a crawling one, their legs, when present, serving more for propulsion than support. The only exceptions to strictly arboreal, terrestrial or aquatic habits among living reptiles are found in the curious and beautiful little flying lizards, or 'flying dragons,' of the Malayan region. In these reptiles, the flattened body is provided with a broad, wing-like expansion of the skin of its sides, supported by the elongate, movable ribs, and capable of being folded up, fan-like. Similar membranous expansions are also found on the sides of the throat. By these means the creature, which lives for the most part among the tree tops, rarely descending to the ground, is capable of certain aerial movements, though not of true flight. In describing its habits a writer has said: "As the lizard lies in shade along the trunk of a tree, its colors at a distance appear like a mixture of brown and gray and render it scarcely distinguishable from the bark. There it remains with no sign of life, except the restless eyes, watching passing insects, which, suddenly expanding its wings, it seizes with a sometimes considerable, unerring leap." Their flight through the air is very swift, so swift that the expansion of the parachute-like membrane may almost escape notice. As in the flying fishes, flying lemurs and flying squirrels, there is not true flight—a power possessed by no living back-boned animals except birds and bats.
Altogether the direct antecedents of the reptiles now living, that is the crocodiles, turtles, lizards and snakes of the past, took no important part in the great Age of Reptiles. They have existed all these millions