Coal; its history and uses . aceArchegosaurus and its kindred prominently forward inthat landscape which was once entirely filled by Lepido-dendron, Sigillaria, and the Calamites. The two occipital condyles and the branchial archeshave been mentioned as characters which unite theLabyrinthodonts to Amphibia, and decisively separatethem from Eeptiles. A third noteworthy feature is the ANIMALS OF THE COAL MEASURES. 129 presence in all Labyrintlioclont skulls of a parasphenoidossification in the roof of the mouth (fig. 23, par.). Sucha bone is universal in Amphibia and in all fishes whichhave an o
Coal; its history and uses . aceArchegosaurus and its kindred prominently forward inthat landscape which was once entirely filled by Lepido-dendron, Sigillaria, and the Calamites. The two occipital condyles and the branchial archeshave been mentioned as characters which unite theLabyrinthodonts to Amphibia, and decisively separatethem from Eeptiles. A third noteworthy feature is the ANIMALS OF THE COAL MEASURES. 129 presence in all Labyrintlioclont skulls of a parasphenoidossification in the roof of the mouth (fig. 23, par.). Sucha bone is universal in Amphibia and in all fishes whichhave an osseous skull; it never occurs as a separate bonein the higher vertebrata, and is nearly always either ru-dimentary or absent. The absence of fin-rays and thedisposition of the bones of the limbs into segments com-parable to those of a Eeptile, Bird, or Mammal, are thefeatures which most clearly cut Labyrinthodonts off fromfishes. Among Amphibia no other known forms exhibittooth-structure like that of the Labyrinthodonts or a. Fig. 23. Under surface of skull of Frog, jiar, parasphenoid (aiter Parker). ventral armour, or an orbital ring like that of Archego-saurus. The zoological place of Labyrinthodonts issatisfactorily determined by these considerations; theyform a peculiar and extinct order of the class Amphibia,to be ranked side by side with the recent orders ofBatrachia (frogs and toads), Urodela (salamanders, newts,&c.), and Ophiomorpha (Ccecilise). Some writers, and among these the eminent palaeonto-logist Hermann von Meyer, have attached classifieatoryimportance to the points of likeness between the Laby-rinthodonts and the Crocodiles. In the larger Laby- 130 COAL. CHAP. IT. rinthodonta, as in Crocodiles, the skull is covered with apitted sculpture, which is probably connected with theattachment and nutrition of the tougli, leathery , the skull of a Labyrinthodont appears to be fuUyossified (at all events, externally) in very young interspaces or fon
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