We may consider the birds at the same time as the sea animals or sea reptiles, since they, perhaps, were relations. Moreover, while the birds of Jurassic times were land birds, those of the Chalk period were aquatic. These birds belonged to two widely different classes, one consisting of large birds which did not fly, the other of small birds with great strength of wing and great powers of flight. Of the first kind was the _Hesperornis_. This was a large flightless bird, specially adapted to diving. Its wings hardly existed, for they had only one bone left; and that implies the passage of a very long flight of time, during which the wings once in existence had become more and more useless, till they had dwindled to a mere nothing. But the _Hesperornis_ had enormously strong legs, which were used as paddles, and their efficiency was increased by the bones of the foot being so joined to the leg as to turn edgewise in the water when brought forward. Any one who has ever paddled a Canadian canoe will appreciate the advantage of this. But this was not all, for the legs were so joined to the body-frame as to stand out nearly at right angles (like a pair of oars), instead of standing under the body as walking legs do. Apparently walking as well as flying had been abandoned, and this bird had become a diver and swimmer merely. The head, neck, and body were long, and admirably shaped for plunging through the water. Favoured by the powerful hind limbs, the _Hesperornis_ must have been very swift both on and under the water, and a formidable enemy to the fishes on which it preferred to feed. Its jaws were armed with teeth set in a groove, and, like the jaws of snakes, were separable so as to admit large prey. As these strange birds were sometimes six feet long, they must have been able to account for fish and reptiles of considerable size. They probably lived nearly altogether on and in the water.