Geology . Fig. 414.—Champsosaurus, from the Laramie of Montana. Length, about six feet. (After Brown.) rise to a group of aquatic reptiles, by some considered of ordinal rank(Choristodera), represented in Europe and in North America by twoclosely allied forms, Simoedosaurus and Champsosaurus (Fig. 414). 182 C LOGY. The latter began in the Laramie epoch, and continued into the Eocene;the former is known only from the Lower Eocene. The sea birds. In the long interval between the first known appear-ance of birds in the Jurassic, and the later Cretaceous, when they re-ap-peared, important chan
Geology . Fig. 414.—Champsosaurus, from the Laramie of Montana. Length, about six feet. (After Brown.) rise to a group of aquatic reptiles, by some considered of ordinal rank(Choristodera), represented in Europe and in North America by twoclosely allied forms, Simoedosaurus and Champsosaurus (Fig. 414). 182 C LOGY. The latter began in the Laramie epoch, and continued into the Eocene;the former is known only from the Lower Eocene. The sea birds. In the long interval between the first known appear-ance of birds in the Jurassic, and the later Cretaceous, when they re-ap-peared, important changes look place, among which was the loss ofthe elongate, bilaterally feathered tail. The Jurassic birds were ter-. Fig. 415.—Hesperornis regalis. Skeleton in U. S. National Museum from whichthe restoration Fig. 415a was made. Sternum and two anterior cervicals sup-plied by restoration. (Lucas.) restrial, while the Cretaceous were aquatic. The Cretaceous birdsbelonged to two widely divergent classes, the one consisting of largeflightless birds (Hesperornis), the other of small birds of powerfulflight (Ichthyornis). The Hesperornis (Fig. 415), was a large, flightless,highly specialized diver, with aborted wings and remarkable leg develop-ment. The wings had almost vanished, a single bone only being implies that, following the evolution which had produced thewings, there was a degenerative history long enough for them to dwindle THE CRETACEOUS PERIOD. 183 almost to the point of extinction. Concurrent with this, and doubt-less its cause, was an extraordinary development of the legs by whichthey became not only very powerful, but their efficiency as paddleswas increased by the bones of the
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