The Ichthyosaurus was admirably formed for cleaving the waves of an agitated sea; but the Plesiosaurus was equally well organised for pursuing its prey in shallow creeks and bays defended from heavy breakers. Its long swan-like neck no doubt enabled it to drag many a victim from its hiding-place. While these huge lizards were the terror of the seas, the Pterodactyles, a race of winged lizards, armed with long jaws and sharp teeth, hovered in the air. With the exception of the greatly elongated fifth finger, to which, as well as to the whole length of the arm and body, the membranous wing or organ of flight was attached, the fingers of this strange animal were provided with sharp claws, so that it was probably enabled, like the bat, to suspend itself from precipitous rock-walls. It is a remarkable fact, that, whereas the Pterodactyles of the older Lias beds did not exceed ten or twelve inches in length, the later forms, found fossil in the Greensand and Wealden beds of the Lower Cretaceous formation, must have been at least 16½ feet long. That these reptiles were not the only vertebrated animals capable of hovering in the air at the time when the huge Ichthyosaurus was lord of the seas, is proved by a bird about the size of a rook, which was discovered in 1862, in the lithographic slate of Solenhofen in Bavaria, a stone-bed belonging to the period of the Upper Oolite. The skeleton of this valuable specimen, now in the British Museum, is almost entire, with the exception of the head, and retains even its feathers. Still older fossil mammalia have been found near Stuttgard, in the uppermost bed of the Triassic deposits, and in the Lower Oolite of Oxfordshire. These interesting remains, which carry back the existence of the mammals to a very remote period, belong to small marsupial, or opossum-like, animals. The jaws, which are the principal parts preserved, are exceedingly minute, and remarkable for the number and distribution of their teeth, which prove them to have been either insectivorous or rodent. The remains of the Ichthyosauri and Plesiosauri occur chiefly in the Liassic group, but the more recent Cretaceous (Wealden) formation is distinguished by the presence of still more enormous land saurians. On their massive legs and unwieldy feet these monsters stood much higher than any reptile of our days, and resembled in bulk and stature the elephants of the present world.