The fish-bed of the Upper Ludlow Rock abounds more in osseous remains than an ancient burying-ground. The stratum, over wide areas, seems an almost continuous layer of matted bones, jaws, teeth, spines, scales, palatal plates, and shagreen-like prickles, all massed together, and converted into a substance of so deep and shining a jet color, that the bed, when "first discovered, conveyed the impression," says Mr. Murchison, "that it enclosed a triturated heap of black beetles." And such are the remains of what seem to have been the first existing vertebrata. Thus, ere our history begins, the existences of two great systems, the Cambrian and the Silurian, had passed into extinction, with the exception of what seem a few connecting links, exclusively molluscs, that are found in England to pass from the higher beds of the Ludlow rocks into the Lower or Tilestone beds of the Old Red Sandstone.[AZ] The exuviæ of at least four platforms of being lay entombed furlong below furlong, amid the gray, mouldering mudstones, the harder arenaceous beds, the consolidated clays, and the concretionary limestones, that underlay the ancient ocean of the Lower Old Red. The earth had already become a vast sepulchre, to a depth beneath the bed of the sea equal to at least twice the height of Ben Nevis over its surface. [Footnote AZ: "Upwards of eight hundred extinct species of animals have been described as belonging to the earliest, or Protozoic and Silurian period, and of these only about one hundred are found also in the overlying Devonian series; while but fifteen are common to the whole Palæozoic period, and not one extends beyond it."--(_M. de Verneuil and Count D'Archiac_, quoted by Mr. D. T. Ansted. 1844.)]