Geology . -try within these regions, it is inferred that they were immigrants fromsome other originating tract, and this tract will doubtless be discoveredin time as the study of other regions progresses. In a minor way, the general coast tract of all the continents, thoughnarrow, was doubtless the originating tract of some species, and per-haps of minor faunas. It is scarcely necessary to remark that the pelagic and abysmallife of the main ocean is not embraced in this review, and is practicallyunknown. General nature of the faunal change.—In nearly all the Paleozoicfaunas, the brachiopods we
Geology . -try within these regions, it is inferred that they were immigrants fromsome other originating tract, and this tract will doubtless be discoveredin time as the study of other regions progresses. In a minor way, the general coast tract of all the continents, thoughnarrow, was doubtless the originating tract of some species, and per-haps of minor faunas. It is scarcely necessary to remark that the pelagic and abysmallife of the main ocean is not embraced in this review, and is practicallyunknown. General nature of the faunal change.—In nearly all the Paleozoicfaunas, the brachiopods were a leading element, while the trilobites,crinoids, corals, and orthoceratites, each in turn, gave distinctivecharacter to the successive faunas. In the Mesozoic era the ammon-ites took the first place, followed by the pelecypods and the ammonites (Fig. 344) were peculiarly fitted for distinguishingsuccessive horizons, not onlv because they were free forms, measur- THE TRIASSIC PERIOD. 51.
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