Towards the close of the Upper Silurian sub-period, the argillaceous beds pass upwards into more sandy and shore-like deposits, in which the most ancient known fossil Fishes occur, and then usher us into the first great ichthyic period of the Old Red Sandstone, or Devonian, so well marked by its fossil fishes in Britain, Russia, and North America. The so-called fish-bones have been the subject of considerable doubt. Between the Upper Ludlow rocks opposite Downton Castle and the next overlying stratum, there occurs a thin bed of soft earthy shale, and fine, soft, yellowish greenstone, immediately overlying the Ludlow rock: just below this a remarkable fish-deposit occurs, called the Ludlow bone-bed, because the bones of animals are found in this stratum in great quantities. Old Drayton treats these bones as a great marvel:-- “With strange and sundry tales Of all their wondrous things; and not the least in Wales, Of that prodigious spring (him neighbouring as he past), That little fishes’ bones continually doth cast.” POLYOLBION. Above the yellow beds, or Downton sandstone, as they are called, organic remains are extensively diffused through the argillaceous strata, which have yielded fragments of fishes’ bones (being the earliest trace yet found of vertebrate life), with seeds and land-plants, the latter clearly indicating the neighbourhood of land, and the poverty of numbers and the small size of the shells, a change of condition in the nature of the waters in which they lived. “It was the central part only,” says Sir R. Murchison, “of this band, or a ginger-bread-coloured layer of a thickness of three or four inches, and dwindling away to a quarter of an inch, exhibiting, when my attention was first directed to it, a matted mass of bony fragments, for the most part of small size and of very peculiar character. Some of the fragments of fish are of a mahogany hue, but others of so brilliant a black that when first discovered they conveyed the impression that the bed was a heap of broken beetles.”[39] [39] “Siluria,” p. 148. [Illustration: Fig. 26.--Halysites catenularius.] [Illustration: Fig. 27.--Pterygotus bilobatus.] The fragments thus discovered were, after examination on the spot, supposed to be those of fishes, but, upon further investigation, many of them were found to belong to Crustaceans. The ichthyic nature of some of them is, however, now well established. [Illustration: Fig. 28.--Plants of the Palæozoic Epoch.--1 and 2, Algæ; 3 and 4, Lycopods.]