File:Geology (1907) (14589755580).jpg

Identifier: geology00cham (find matches)
Title: Geology
Year: 1907 (1900s)
Authors: Chamberlin, Thomas C. (Thomas Chrowder), 1843-1928 Salisbury, Rollin D., 1859- joint author
Subjects: Geology
Publisher: New York, H. Holt and co.
Contributing Library: Internet Archive
Digitizing Sponsor: Internet Archive

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the whole family seems yet to have been confined to North America.The oreodons, though destined to become extinct at the close of theperiod, were represented by 18 American species. They appear thusnot to have dwindled away but to have gone out suddenly, in thegeological sense, not unlikely from the attacks of some new carnivore.They appear never to have migrated from North America. The pec-caries do not seem to have been specially abundant. The evolution of the horse.—It was a great epoch in the evolution ofthe horse, Anchippus, Protohippus, Pliohippus (Merychippus), Hij>parion, 1 Bull. Am. Mus. Nat. Hist., XII, 1899, p. 74. THE M IOC EXE PERIOD. 287 and other genera flourished and deployed into forty or more species.They were still three-toed, but the two lateral toes were much reducedand did not usually touch the ground, while the central one wasstrengthened and bore all the weight. A large group of structuralfeatures were being modified, concurrently with the feet, to fit the
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Fig. 455.—An American Miocene Camel, Oxydactylus longipes Peterson, from theLoup Fork beds of Nebraska. (After Peterson.) evolving horse to the open dry plains and their grassy food (Fig. 456).The elimination of the side toes, the lengthening of the limbs, thechange of the joints to the pulley-wheel type, the concentrationof the limb muscles near the body to reduce the weight of the partsmost moved, and the consolidation of the leg bones, were modifica-tions in the interest of combined speed and strength. A correspondingelongation of head and neck was necessary to reach the ground. Thefront teeth were reduced to chisel-like, cropping forms, somewhatresembling those of the rodents, while the molars evolved a tortuousdistribution of the enamel so flanked by dentine and cement thatthe differences of wear gave rise to ridges of enamel suited to grinding,and protected against breaking by supporting dentine and cementon either side. The teeth were also gradually elongated to provide 288 G

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