Archelon was a big, beautiful turtle. A sea turtle, a turtle of the sea, from many millions of years ago, towards the tail end of the time when dinosaurs (of which Archelon was pointedly not one) roamed the Earth.

In size, Archelon has often aptly been compared to an automobile. And not a small car either, not one of those minis tooling around the roads of Europe, but a good-sized vehicle, perhaps an unusually wide and round station wagon comfortably carting around a family of five, and their dog. Archelon was in fact wider than he was long, up to thirteen feet from nose to tail, but having up to a sixteen foot wingspan. In visage, Archelon most closely resembled the leatherback turtle of today, with its rock-hard skin and vaulted rows of bones down the back substituting for a proper fully-plated turtle shell.

And how wing-like those flippers were!! All indications are that Archelon was a very happy species of prehistoric turtle, which was able despite its vast size to do freewheeling cartwheels in the deep open waters of Earth's ancient oceans. If you were to come upon Archelon you would likely wish to feed it a snack and pet it. And you probably need not fear that Archelon would eat you for a snack. For though Archelon was omnivorous, its animal prey tended to be on the small side, only what it could snap up in one bite. But, in that vein of thought, Archelon might eat a baby. Naturally, you would wish to ride on Archelon's back like a prince or princess of the sea (along with your family of five and your dog, all of whom would fit comfortably there, though you must keep a careful eye on the baby). Such is the charm and personality of Archelon.

Archelon!! O, Archelon!!
Wherefore art thou, O Archelon?
Thy kingdom come, thy shell shine on
In the Cretaceous, as in the Triassic.

Further reading:

The BBC info page on Archelon