Geology Volume 2

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1907 edition. Excerpt: ...to one half, and in later periods declined still further until they disappeared at the close of the Paleozoic era. The rise and fall of the trilobites is shown graphically in the curve of the accompanying figure (Fig. 156). Their climax appears to have been reached by a rapid evolutionary ascent, and to have been followed by a more gradual decline, but the ascent was doubtless very much more prolonged in reality than appears from the record, which is doubtless less complete in the Cambrian and pre-Cambrian systems than in those of later periods. The attainment of a climax so early in the recorded history of life is a very notable event. The cystoids and the graptolites also reach their climaxes in this period. The cystoids were, however, replaced by their near kin, the crinoids, and the much simpler organization of the graptolites makes 1 Walcott. Bull Geo, Soc. of Am., Vol. Ill, pp. 153-171. their early climax less significant. The climax of the trilobite: therefore be regarded as the foremost great event of its kind t definitely recorded. Fio. 15G.--The two upper curves represent tlie history of the trilobites ac to genera, the full line indicating the total number of genera present in th of the successive periods, and the dotted line the number of new genera inti The two lower curves present the same data for the families of the trilol full line representing the total number of families present, and the dot the number of new families introduced. The data for the families is tak Beeclier in the Zittel-Eastman text-book of Paleontology, Vol. I. The the genera is somewhat incomplete, but is as full as can be made from "Handbuch der Pala: ontologie." The general aspect of the trilobites at the high tide of their is fairly...