Page:The Habitat of the Eurypterida.djvu/160

154 limestone is succeeded by the Mocktree or Dayia navicula shales, which in turn are followed by the Lower Whitcliffe flags with abundant Rhynchonella nucula, Orthis lunata and more rarely Chonetes striatella. The latter fossil becomes dominant in the Upper Whitcliffe flags which, together with the succeeding beds, are well exposed in the famous Ludford Lane now known as the Whitcliffe Road section, near Dinham Bridge. Chonetes striatella "literally swarms" in these flags, and Orbiculoidea rugata and Orthoceras bullatum are likewise prolific. It is these flags which show the first traces of fragments of that little known eurypterid, Pterygotus problematicus, which occurs in the thin shales and sandstones with Spirifera elevata, Chonetes striatella and Orbiculoidea rugata (see fig. 15). The shales and sandstones carrying the fauna just mentioned, are only four feet thick, yet eight changes in sedimentation are shown, marking a rapid alternation of mud and sand deposition which is clearly indicative of near-shore conditions. Immediately overlying this series is the topmost member of the series, the Ludlow Bone-Bed which though never more than a foot in thickness, is yet one of the most noted of the formations of Britain. It has been the subject of description and speculation for seventy-five years or more; but, so far as I know, its origin has never been satisfactorily accounted for (see proposed explanation below, p. 158). Elles and Wood describe the appearance of the Bone-Bed in this section as follows: "It is best developed at the lower end of the section, on the south side of the road where it is 2$$\tfrac{1}{2}$$ feet above road-level, and reaches a maximum thickness of nearly 6 inches. It is, however, very commonly separated into two thin bands of 'bony' material, divided by a few inches of soft mudstone. These bands occur in a more or less lenticular manner, and one or the other disappears almost entirely from time to time, even within the short distance occupied by the section (72 yards). This feature is characteristic of all the bone-beds of these highest Silurian rocks. In addition to the numerous fish-remains and crustacean remains which the Bone-Bed contains, we have identified Chonetes striatella, Orbiculoidea rugata, and Orthis sp: a similar fauna, with Beyrichia in addition, being found in the softer mudstone separating the 'bony layers.' " (61, 203).

Above the Bone-Bed there is a physical and faunal change, the sediments are coarser, sandstones predominating, with only thin interbedded shales, while the genera of brachiopods so characteristic of the strata of the Aymestry and Lower Ludlow have almost vanished