Page:The Habitat of the Eurypterida.djvu/164

156 :Ctenacanthus sp. (?)
 * Cephalaspis sp. (?)
 * Pachytheca sphaerica

The olive shales above the Bone-Bed also contain many fragments of eurypterids. Evidence of the approach of the Old Red sandstone deposition is seen in the frequent occurrence of grit bands in the olive shales. The top of F f is the "Fragment-Bed" which is crowded with fragments of carbonaceous material whose origin is uncertain, and this layer is everywhere succeeded by the purple-red sandstones of the Old Red.

These sections show the typical lithological and faunal characteristics of the Ludlow in England, and they offer unquestionable evidence for a change from marine to continental conditions of sedimentation. Beginning with the Aymestry group which is a pure marine limestone in the lower part, passing up into shales with thin limestone beds, the succession continues through the flags of the Upper Ludlow group, terminated by the Ludlow Bone-Bed, and finally the Temeside group closes the Siluric. These last beds consist of the Downton Castle sandstones in the lower half, which show an alternation of unfossiliferous sandstones and shales with beds of similar character bearing Lingulas or Platyschisma, or eurypterids or fish remains, while the upper portion constitutes the Temeside or Eurypterus-shales which are dominantly eurypterid-bearing, olive shales, with intercalated grit bands, fish beds and bone-beds. In regard to these formations in the Ludlow-Downton district, Elles and Slater make the following significant statement: "Palæontologically, these rocks are characterized by the presence of Eurypteridae, which, although rare in the lower beds, gradually increase in importance until they attain their maximum development in the beds immediately underlying the Old Red sandstone. The rich brachiopod-fauna, characteristic of the lower beds, dwindles and almost dies out with the approach of shallow-water conditions, although the molluscs are somewhat more persistent" (37, 197). The eurypyterids occur in thin seams not associated with the fast diminishing marine fauna, but with crustacea such as Beyrichia, with the thin-shelled Platyschisma helicites and occasional Lingulas, and especially with fishes. The eurypterids are scarce in the Aymestry and Upper Ludlow groups, but become abundant in certain layers in the Temeside group where they are found in cross-bedded sandstones, in bone-beds, and characteristically, in olive-colored shales.