The shell was depressed; the dermal plates covering the ribs had almost entirely disappeared, remnants only of their upper ends remaining; the skull (Fig. 127) had the temporal region wholly roofed over; the neck was short and not retractile. The front legs were strong flippers, the humerus was long and stout, with the crest for the attachment of muscles far down on the shaft; the digits were greatly elongated and clawless, etc. The plastron only was less reduced than in the case of the modern sea-turtles. No traces of horny shields have been discovered. As to the nature of the covering and the general appearance of the turtle when alive, Dr. Wieland has kindly given the writer his views, as follows: “After direct study or fairly close examination of all the fossil material of importance thus far collected representing the Protostegidae, it seems certain that in all the members of the group an external leathery layer was well developed. In no instance is there the slightest trace of horny shield sulci, or grooves; though it seems probable that there was some gradation toward a thin and perhaps even slightly horny hide. In _Archelon ischyros_ the reduced condition of the carapace and the presence of the continuous row of large, median, supraneural elements render it quite certain that there was a development of leathery hide comparable to that of _Dermochelys_. The same may be said of _Protostega gigas_. But _Archelon Marshii_ had a less reduced carapace, and the leathery skin was probably less well developed; and _Protostega Copei_, in which no trace of supraneurals remains, must have made some approach to the horn-shield condition. A more distinct suggestion of transition from the leathery to the horny shield covering may be seen in the very different contemporary Cretaceous form, _Toxochelys Bauri_, where ossified epi-or supraneurals occupy quite exactly the nodal relation of the five vertebral horn shields of later turtles, like _Lytoloma_, though there are not the slightest traces of sulci. [Illustration: FIG. 126.—_Archelon ischyros_, a gigantic sea-turtle from the Upper Cretaceous of South Dakota.] [Illustration: FIG. 127.—Skull of _Archelon ischyros_: _pa_, parietal; _f_, frontal; _pm_, premaxilla; _pf_, prefrontal; _ptf_, postfrontal; _m_, maxilla; _j_, jugal; _qj_, quadratojugal; _sur_, surangular; _d_, dentary; _an_, angular. (After Wieland.)] “From a purely anatomical standpoint I have suggested that _Archelon_ had seven dorsal keels corresponding to those of _Dermochelys_. There is much excellent reason for regarding dermogene ossification as essentially double-layered throughout the Reptilia.