Far out on its expanse might have been seen in those ancient days, a huge snake-like form which rose above the surface and stood erect, with tapering throat and arrow-shaped head; or swayed about, describing a circle of twenty feet radius above the water. Then it would dive into the depths, and naught would be visible but the foam caused by the disappearing mass of life. Should several have appeared together, we can easily imagine tall twining forms, rising to the height of the masts of a fishing fleet, or like snakes twisting and knotting themselves together. This extraordinary neck, for such it was, rose from a body of elephantine proportions; and a tail of the serpent pattern balanced it behind. The limbs were probably two pairs of paddles, like those of _Plesiosaurus_, from which this diver chiefly differed in the arrangement of the bones of the breast. In the best known species, twenty-two feet represent the neck, in a total length of fifty feet. This is the _Elasmosaurus platyurus_ (Cope), a carnivorous sea reptile, no doubt adapted for deeper waters than many of the others. Like the snake-bird of Florida, it probably often swam many feet below the surface, raising the head to the distant air for a breath, then withdrawing it and exploring the depths forty feet below, without altering the position of its body. From the localities in which the bones have been found in Kansas, it must have wandered far from land, and that many kinds of fishes formed its food, is shown by the teeth and scales found in the position of its stomach. A second species, of somewhat similar character and habits, differed very much in some points of structure. The neck was drawn out to a wonderful degree of attenuation, while the tail was relatively very stout, more so, indeed, than in the _Elasmosaurus_, as though to balance the anterior regions while occupied in various actions, _e. g._, while capturing its food. This was a powerful swimmer, its paddles measuring four feet in length, with an expanse, therefore, of about eleven feet. It is known as _Polycotylus latipinnis_ (Cope). The two species just described formed a small representation, in our great interior sea, of an order which swarmed at the same time, or near it, over the gulfs and bays of old Europe. There they abounded twenty to one. Perhaps one reason for this was the almost entire absence of the real rulers of the waters of Ancient America, viz: the _Pythonomorphs_. These sea-serpents, for such they were, embrace more than half the species found in the limestone rocks in Kansas, and abound in those of New Jersey and Alabama. Only four have been seen as yet in Europe.