Coal; its history and uses . erent improbabilityof a tail-less Labyrinthodont with excessively prolongedhind legs must be allowed its full weight. We shall dowell to banish the old wood-cut, at which more than onegeneration of geologists has gazed with satisfaction, andto replace it by another figure—that of a tailed, aquaticanimal, feeble on land, but swift and rapacious in thewater, a Crocodile in outward form and habits, thoughAmphibian in its derivation and internal stracture. The Triassic Labyrinthodonts exhibit more conspi-cuously than those of earlier date the peculiar tooth- CHAP. lY.
Coal; its history and uses . erent improbabilityof a tail-less Labyrinthodont with excessively prolongedhind legs must be allowed its full weight. We shall dowell to banish the old wood-cut, at which more than onegeneration of geologists has gazed with satisfaction, andto replace it by another figure—that of a tailed, aquaticanimal, feeble on land, but swift and rapacious in thewater, a Crocodile in outward form and habits, thoughAmphibian in its derivation and internal stracture. The Triassic Labyrinthodonts exhibit more conspi-cuously than those of earlier date the peculiar tooth- CHAP. lY. ANIMALS OF THE COAL MEASURES. 117 structure which has given a name to the order. TromArchegosaurus we learn that the tooth is first formed asa small hollow cone of enaaiel with two cutting , the true crown of the tooth, retains its original sizeand structure until it disappears by wear or fracture. Itdoes not, however, remain directly attached, as at first,to the dentigerous bone, but is gradually elevated by the. Fig. 16. Section of tooth of Mastodonsaurus (magnified). growth of a continually enlarging base. This base, whichis often the only part of the tooth remaining, has the formof a hollow cone of tooth-substance (dentine), coatedthinly with enamel, and enclosing a pulp-cavity. Its wallbecomes longitudinally and radiately folded, as if frilled,and in the Triassic species some or nearly all of the plaitsmay be again folded tangentially to the surface of thetooth. The folds are ordinarily close together, and leave 118 COAt. only linear spaces between. In this way the thickness ofthe tooth-wall is greatly increased, and the central cavitymuch encroached upon. The enamel may extend itselfbetween the dentine as a thin undulating layer. A crosssection of such a tooth as has been described shows a setof sinuous and branched passages, which originally com-municated with the exterior, but are now obliterated oroccupied by enamel, and a corresponding set (divided fromthe
Size: 1603px × 1559px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1870, bookpublisherlo, booksubjectcoal
