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.]

Of all these strange old forms, so singularly uniting features of Fishes and Reptiles, none has given rise to more discussion than the Pterodactylus, (Figure 3,) another of the Saurian tribe, associated, however, with Birds by some naturalists, on account of its large wing-like appendages.  From the extraordinary length of its anterior limbs, they have generally been described as wings, and the animal is usually represented as a flying Reptile.  But if we consider its whole structure, this does not seem probable, and I believe it to have been an essentially aquatic animal, moving after the fashion of the Sea-Turtle.  Its so-called wings resemble in structure the front paddles of the Sea-Turtles far more than the wings of a Bird; differing from them, indeed, only by the extraordinary length of the inner toe, while the outer ones are comparatively much shorter.  But, notwithstanding this difference, the hand of the Pterodactylus is constructed like that of an aquatic swimming marine Reptile; and I believe, that, if we represent it with its long neck stretched upon the water, its large head furnished with powerful, well-armed jaws, ready to dive after the innumerable smaller animals living in the same ocean, we shall have a more natural picture of its habits than if we consider it as a flying animal, which it is generally supposed to have been.  It has not the powerful breast-bone, with the large projecting keel along the