. Elements of geology : a text-book for colleges and for the general reader. Geology. 436 MESOZOIC ERA—AGE OF REPTILES, Of these, about three fourths are beetles. The earliest of the higher group of insects seem to have been introduced here, although they do not become abundant until the Tertiary. Fishes,—It will be remembered that the Placoids of the Palaeozoic were nearly all Cestracionts, or crushing-toothed sharks. The Hybo-. Fig. 682. Figs. 679-682.—Jurassic Fishes—Placoids: 679. Tooth of Acrodus nobilis. 680. Hybodus reticu- latus, Spine and Tooth. 681. Squatina acanthoderma. 682. Ganoid
. Elements of geology : a text-book for colleges and for the general reader. Geology. 436 MESOZOIC ERA—AGE OF REPTILES, Of these, about three fourths are beetles. The earliest of the higher group of insects seem to have been introduced here, although they do not become abundant until the Tertiary. Fishes,—It will be remembered that the Placoids of the Palaeozoic were nearly all Cestracionts, or crushing-toothed sharks. The Hybo-. Fig. 682. Figs. 679-682.—Jurassic Fishes—Placoids: 679. Tooth of Acrodus nobilis. 680. Hybodus reticu- latus, Spine and Tooth. 681. Squatina acanthoderma. 682. Ganoid: Tetragonolepis, restored, and Scales of the same. donts, or sharks with teeth pointed, but rounded on the edges, com- menced in the Carboniferous and increased in the Triassic. Now, in the Jurassic the Cestracionts continue (Fig. 679), but in diminished numbers. The Hybodonts culminate (Fig. 680), and the Squalodonts, or modern sharks, with lancet-shaped teeth, commence in small num- bers. Rays (Fig. 681), which maybe regarded as among the highest of Placoids, are found in considerable numbers in the Jurassic. Ganoids continue, but take on far more modern forms, and have now in most cases lost the vertebrated structure of the tail-fin, thus foreshadowing the Teleosts, which appear in the next period. Among the most characteristic Ganoids of this period, and, in fact, of this age, are the Pycnodonts, a family characterized by a broad, flat body, rhom-. 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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