. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 321. Fig. 322. Figs. 321 and 322.—Silurian Polyzoa: 321. Fenestella elegans (after Hall). 322. Alecto aulopo- (after Hall). Echinoderms may be conveniently divided into three orders, viz.: the Echinoids, or sea-urchins; the Asteroids, or star-fishes; and the Cri- noids. The members of the first and second orders are free moving, while those of the third are stemmed. Of these orders the Crinoids are the lowest, as proved not only by their simpler organization, but also by the fact that a living Cr
. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 321. Fig. 322. Figs. 321 and 322.—Silurian Polyzoa: 321. Fenestella elegans (after Hall). 322. Alecto aulopo- (after Hall). Echinoderms may be conveniently divided into three orders, viz.: the Echinoids, or sea-urchins; the Asteroids, or star-fishes; and the Cri- noids. The members of the first and second orders are free moving, while those of the third are stemmed. Of these orders the Crinoids are the lowest, as proved not only by their simpler organization, but also by the fact that a living Crinoid, the Comatula (Fig. 325), is attached when young, but free when mature. Now, in Silurian times, the stemmed Echinoderms are very abundant, while the free are very rare : at the present Fig. 323. Fig. 324. J r Figs. 323 and 324.—Living Crinoids: 323. Rhizocrinus time, On the Contrary, the re- CapuSdia"er Thompson)- 324' Pentacrhllls verse is the case. Thus, in the course of time, the former decreased until they are now almost extinct, while the latter increased until they are now very abundant. If we take the abundance of Echino- derms during geological times as constant, and represent the course of time by the absciss A B (Fig. 326), and the abundance by distance from A B to C D, then the parallelogram would represent this Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original LeConte, Joseph, 1823-1901. New York : D. Appleton and Co.
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