154 T HE HABITAT OF THE EURYPTERIDA 



sive limestone is succeeded by the Mocktree or Dayia navicula shales, 

 which in turn are followed by the Lower Whitcliffe flags with abun- 

 dant Rhynchonella nucula, Orthis lunata and more rarely Chonetes 

 striatella. The latter fossil becomes dominant in the Upper Whit- 

 cliffe flags which, together with the succeeding beds, are well exposed 

 in the famous Ludford Lane now known as the Whitcliffe Road sec- 

 tion, near Dinham Bridge. Chonetes striatella "literally swarms" in 

 these flags, and Orbiculoidea rugata and Orthoceras bullatum are like- 

 wise prolific. It is these flags which show the first traces of frag- 

 ments 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 alter- 

 nation 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 descrip- 

 tion 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\ 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 fauna! change, the 

 sediments are coarser, sandstones predominating, with only thin inter- 

 bedded shales, while the genera of brachiopods so characteristic of 

 the strata of the Aymestry and Lower Ludlow have almost vanished 



