Another animal which lived at this period was the Plesiosaurus. It lived in shallow seas and estuaries, and would seem, from its organs of respiration, to have required frequent supplies of fresh air. Mr. Conybeare describes it as "swimming upon, or near the surface, arching its long neck like the swan, and occasionally darting it down at the fish which happened to float within its reach." This reptile, which was smaller than the Ichthyosaurus, has been found as long as from twelve to fifteen feet. Its appearance and habits differed from the latter materially. The Ichthyosaurus, with its short neck, powerful jaws, and lizard-like body, seems admirably suited to range through the deep waters, unrivaled in size or strength, and monarch of the then existing world; the Plesiosaurus, smaller in size and inferior in strength, shunned its powerful antagonist, and, lurking in shallows and sheltered bays, remained secure from the assaults of its dangerous foe, its long neck and small head being well adapted to enable it to dart on its prey, as it lay concealed amid the tangled sea-weed. This has been called by geologists the "age of reptiles;" their remains are found in great numbers in the lias, oolite, and wealden strata. These creatures seem to form a connecting link between the fishes of the previous era, and the mammalia of the Tertiary age; the Ichthyosaurus differed little from a fish in shape, and its paddles or feet are not unlike fins, the Plesiosaurus, on the contrary, as its name denotes, partook more of the quadruped form. Dr. Buckland in describing it, says: "To the head of a lizard it united the teeth of a crocodile; a neck of enormous length, resembling the body of a serpent; a trunk and tail having the proportions of an ordinary quadruped; the ribs of a cameleon, and the paddles of a whale." Besides these animals we find the Pterodactyle, half bird and half reptile; the Megalosaurus, or gigantic lizard; the Hylæosaurus, or forest lizard; the Geosaurus, or land lizard, and many others, all partaking more or less of affinity to both the piscatory and saurian tribes.