. The chordates. Chordata. 460 Comparative Morphology of Chordates. Fig. 356. Cacops, a small labyrinthodont amphibian from the Lower Permian. Original about 16 inches long. (After Williston. Courtesy, Romer: "Vertebrate Paleontology," Chicago, University of Chicago Press.) amphibians were more or less extensively covered by dermal bony scales, especially on the ventral surface of the body. The teeth had a peculiarly complex internal structure (Fig. 41), because of which the group is named Labyrinthodontia. Because of the cephalic covering by extensive plates of dermal bone (enlarged
. The chordates. Chordata. 460 Comparative Morphology of Chordates. Fig. 356. Cacops, a small labyrinthodont amphibian from the Lower Permian. Original about 16 inches long. (After Williston. Courtesy, Romer: "Vertebrate Paleontology," Chicago, University of Chicago Press.) amphibians were more or less extensively covered by dermal bony scales, especially on the ventral surface of the body. The teeth had a peculiarly complex internal structure (Fig. 41), because of which the group is named Labyrinthodontia. Because of the cephalic covering by extensive plates of dermal bone (enlarged scales), it is also called Stegocephalia (Fig. 356). In size, many of the old amphibians greatly exceeded even our modern 5-foot salamander of eastern Asia, some being probably as much as 15 feet long. Others, however, were of the size of modern salamanders and newts. In the course of the later history of the Class, the pectoral girdle lost connection with the skull and the pelvic girdle acquired connection with a sacral vertebra, thereby increasing the efficiency of the hind- legs in pushing the body forward. The scaly armor of the body was all but lost, the labyrinthine pattern of the teeth vanished, the aver- age size of the animals was much reduced, and the group differentiated into the types represented by the three existing orders. This later his- tory is known in only a very fragmentary way because of scarcity of fossil evidence. No fossil caecilians (Apoda) have ever been found. The early amphibians were contemporary with crossopterygian ("lobe-fin") fishes whose beginnings were considerably before (early Devonian) those of amphibians. In structure of skull and in other skeletal features, and especially in the possession of an armor of cal- careous scales, there is much resemblance between the early amphib- ians and the fishes. The skulls of the fishes give evidence of connection between nasal cavities and mouth (internal nares), which implies possession of l
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