The genus holoptychius, which began in the old red sandstone, again occurs in the carboniferous system, under eight new specific forms. Along with the megalichthys, afterward noticed, these constitute the two great natural families of fishes of carnivorous propensities, which give a marked character to the period. The prodigious increase of the shark-like creatures, of which not less than sixty species have been described from thousands of teeth, fins, detached vertebræ, and other fragments, is equally striking. Thus, in all, the faunæ of the carboniferous period amount to upward of a thousand species, which have been either figured or described. In contemplating the period of creation under review, we are struck not more with the forms of life which actually existed, than with the absence of races which were afterward so abundant. No quadruped or true terrestrial animal is found so low in the series of rocks, or mixed up in any way with all this profusion of marine exuviæ. Fossil insects and indications of other winged tribes have been detected; but no bone nor foot-print of beast, or inhabitant of land, has anywhere been discovered. The fact is all-important, as showing not only a plan, but a progress and succession in the work of creation. A vegetation, so rank and luxuriant as has been traced, trees towering hundreds of feet into the sky, and branches of the densest foliage stretching on every side, was amply fitted to afford shelter and food to families of terrestrial creatures of every kind. But in the circumstance, that during this period there were repeated alternations of marine and fresh water deposits, and consequently repeated submergence and elevation of land, we see a reason why the terrestrial races were not yet called into being. Great continents, comparatively speaking, did not exist; and there was no ark of safety provided to float them over the billows. Race after race would have violently perished during every shift or subsidence of the sea bottom: and hence, until the carboniferous series was completed and a statical equilibrium established between the land and waters, few or none of the races which afterward swarmed in our plains and forests were introduced upon the scene. [Illustration: Fragment of Encrinital Limestone.] CHAPTER VIII. GEOGRAPHICAL DISTRIBUTION OF COAL—GREAT COAL FIELD OF PENNSYLVANIA, VIRGINIA AND OHIO—COAL DEPOSITS OF ILLINOIS, INDIANA AND KENTUCKY—ECONOMIC HISTORY—CONDITIONS OF FORMATION.