. The evolution of the earth and its inhabitants; a series delivered before the Yale chapter of the Sigma xi during the academic year 1916-1917. ans also exist, as the Africanlung-fish, Protopterus, of the Nile, which, during times ofdrought, forms for itself a cocoon-like case of slime-hardened 4 See Barrell, J., Influence of Silurian-Devonian Climates on the Rise of Air-breathing Vertebrates. Bull. Geol. Soc. America, vol. 27, 1916, pp. 387-436. 122 EVOLUTION OF THE EARTH mud in the river bottom and aestivates therein; and the SouthAmerican lung-fish, Lepidosiren, of the Amazon River and its
. The evolution of the earth and its inhabitants; a series delivered before the Yale chapter of the Sigma xi during the academic year 1916-1917. ans also exist, as the Africanlung-fish, Protopterus, of the Nile, which, during times ofdrought, forms for itself a cocoon-like case of slime-hardened 4 See Barrell, J., Influence of Silurian-Devonian Climates on the Rise of Air-breathing Vertebrates. Bull. Geol. Soc. America, vol. 27, 1916, pp. 387-436. 122 EVOLUTION OF THE EARTH mud in the river bottom and aestivates therein; and the SouthAmerican lung-fish, Lepidosiren, of the Amazon River and itsaffluents, which lives near the margin of the water, using itslung almost with the regularity of a mammal, and also forminga burrow for its habitation during the dry season. Of thesedipnoans the latter two belong to a group of which we haveno fossil record; Neoceratodits, on the other hand, is a relicof what was formerly a large and widespread group. Yetanother order of fishes in which the air-bladder has a respira-tory value is the Crossopterygii, or fringe-finned ganoids, againan important and numerous group in the geologic past but now. FIG. 19.—African fringe-finned ganoid, Polypterus delhezi. After Jordan, fromLulls Organic Evolution, published by the Macmillan Company. represented by but two genera, Polypterus (Fig. 19) andErpetoichthys, both tropical African in distribution. Whilethese living genera have not so effective a respiratory deviceas the dipnoans, nevertheless they present fewer anatomicaldifficulties to stand in the way of relationship with the am-phibia. Without rehearsing the technical arguments, it maysuffice to say that the generally accepted view is that the terres-trial vertebrates were derived either from ancient crossop-terygians or from a group ancestral to both them and geologic cause which lies back of the emergence isapparent. Diastrophic movement during the Silurian period(see Fig. 14) initiated a widespread aridity which culminated
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