“In the very young tortoise or turtle the ribs are separate, as in other animals. As they grow older they begin to expand at the upper side of the upper end, and with increased age the expansion extends throughout the length. The ribs first come in contact where the process commences, and in the land tortoise they are united at the end. In the sea-turtles the union ceases a little above the ends. The fragments of the Protostega were seen by one of the men projecting from a ledge of a low bluff. After several square feet of rock had been removed, we cleared up the floor and found ourselves well repaid. Many long, slender pieces of two inches in width lay upon the ledge. They were evidently ribs, with the usual heads, but behind each head was a plate-like the flattened bowl of a huge spoon, placed crosswise. Beneath these stretched two broad plates, two feet in width, and no thicker than binder’s board. The edges were fingered and the surface was hard and smooth. All this was quite new, among fully grown animals. Some bones of a large paddle were recognized, and a leg bone. The shoulder-blade of a huge tortoise came next, and further examination showed that we had stumbled on the burial place of the largest species of sea-turtle yet known. But the ribs were those of an ordinary turtle just hatched, and the great plates represented the bony deposit in the skin, which, commencing independently in modern turtles, unite with each other at an early day. But it was incredible that the largest of known turtles should be but just hatched, and for this and other reasons it has been concluded that this ‘ancient mariner’ is one of those forms, not uncommon in old days, whose incompleteness in some respects points to the truth of the belief that animals have assumed their modern perfection by a process of growth from more simple beginnings.”
Later studies by Doctors G. Baur, E. C. Case, O. P. Hay, and especially G. R. Wieland, of the abundant and excellent material, preserved in the museums of Yale and Kansas universities and the Carnegie Institution, and especially the discovery by Wieland in 1895 of an allied and yet larger form which he called Archelon, have determined practically every detail of the structure of this remarkable group of sea-turtles. A surprisingly complete specimen of Archelon is mounted in the museum of Yale University.
Fig. 123.—Archelon ischyros; skeleton from above: n, nuchal, r, r, r, ribs; m, m, peripheral bones; h, humerus; r, radius; u, ulna; t, tibia; fi, fibula. (From Wieland.)
About a half-dozen species and two genera of the family have so far been described, all coming from the Upper Cretaceous deposit of Kansas and South Dakota, the genus Archelon from later rocks than those which have yielded Protostega.
The general form and structure of Archelon will best be understood from the accompanying figures after Wieland ([Figs. 123], [124], [125]) and the restoration of the living animal as interpreted by the writer ([Fig. 126]). If the leather-back turtle, described farther on, is really the descendant of these or allied turtles, as many authors believe, it of course represents the very highest aquatic specialization of all Chelonians. If, on the other hand, as some believe, the leather-back is the end of a long and independent line of descent, then Archelon represents the highest aquatic specialization of all other turtles.
In size, at least, Archelon attained the maximum of the order, reaching a length of more than twelve feet, and a weight of more than three tons. Except that the shell was not heart-shaped or elongated as in all modern sea-turtles, but nearly circular in outline, it had all the aquatic adaptations of the sea-turtle in a yet higher degree.
Fig. 124.—Archelon from below, without plastron: h, humerus; r, radius; u, ulna; sc, scapula; c, coracoid; p, pubis; i, ischium. (From Wieland.)
