Another most curious member of this group is the Plesiosaurus, odd Saurian (Figure 2). By its disproportionately long and flexible neck, and its small, flat head, it unquestionably foreshadows the Serpents, while by the structure of the backbone, the limbs, and the tail, it is closely allied with the Ichthyosaurus. Its flappers are, however, more slender, less clumsy, and were, no doubt, adapted to more rapid motion than the fins of the Ichthyosaurus, while its tail is shorter in proportion to the whole length of the animal. It seems probable, from its general structure, that the Ichthyosaurus moved like a Fish, chiefly by the flapping of the tail, aided by the fins, while in the Plesiosaurus the tail must have been much less efficient as a locomotive organ, and the long, snake-like, flexible neck no doubt rendered the whole body more agile and rapid in its movements. In comparing the two, it may be said, that, as a whole, the Ichthyosaurus, though belonging by its structure to the class of Reptiles, has a closer external resemblance to the Fishes, while the Plesiosaurus is more decidedly reptilian in character. If there exists any animal in our waters, not yet known to naturalists, answering to the descriptions of the "Sea-Serpent," it must be closely allied to the Plesiosaurus. The occurrence in the fresh waters of North America of a Fish, the Lepidosteus, which is closely allied to the fossil Fishes found with the Plesiosaurus in the Jurassic beds, renders such a supposition probable. [Illustration: Fig. 3. A Pterodactylus.]