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. [Illustration: 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. [Illustration: FIG. 124.—_Archelon_ from below, without plastron: _h_, humerus; _r_, radius; _u_, ulna; _sc_, scapula; _c_, coracoid; _p_, pubis; _i_, ischium. (From Wieland.)] [Illustration: FIG. 125.—_Archelon_; skeleton from below: _hp_, hyoplastron; _hpp_, hypoplastron. (From Wieland.)]