In Fig. 38 on p. 85 are shown two paddles, the front and hind paddles of a single individual of a very specialized plesiosaur from the Upper Cretaceous of Kansas (_Trinacromerum_). The long arm and thigh bones are followed by remarkably short and broad bones in place of the elongated forearm and leg bones of the land reptiles. Not only are these bones much broader than they are long, but there have been developed additional bones back of them in the same row—new bones which have no counterpart in any terrestrial reptiles. In the first of the three figures is shown a hind paddle of one of the earliest known plesiosaurs, _Thaumatosaurus_, from the lower part of the Jurassic of Germany. It will be seen here that the tibia and fibula are much more elongated than in _Trinacromerum_, and much more like the leg bones of land reptiles. A still more primitive stage in the evolution of the swimming paddle of the plesiosaurs will be seen in Fig. 48 on p. 99, the possibly ancestral, amphibious nothosaur. Here the tibia and fibula, while relatively very much shorter than in any land reptile, still have, together with all the other bones of the leg, a terrestrial or amphibious type. In Fig. 39 is seen the front paddles of the long-necked _Elasmosaurus_, which, though one of the latest of all plesiosaurs in geological history, has the structure of its paddles somewhat intermediate between that of the earlier _Plesiosaurus_ and the later _Trinacromerum_. [Illustration: FIG. 40.—Skull of _Elasmosaurus_ from the side: _pm_, premaxilla; _m_, maxilla; _po_, postorbital; _j_, jugal.]