156 THE HABITAT OF THE EURYPTERIDA 



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 characteris- 

 tics of the Ludlow in England, and they offer unquestionable evi- 

 dence for a change from marine to continental conditions of sedimen- 

 tation. Beginning with the Aymestry group which is a pure marine 

 limestone in the lower part, passing up into shales with thin lime- 

 stone beds, the succession continues through the flags of the Upper Lud- 

 low group, terminated by the Ludlow Bone-Bed, and finally the Teme- 

 side group closes the Siluric. These last beds consist of the Down- 

 ton 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 inter- 

 calated 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:. " Palaeontologically , 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 shal- 

 low-water conditions, although the molluscs are somewhat more per- 

 sistent" (37, 197). The eurypyterids occur in thin seams not asso- 

 ciated 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 abun- 

 dant in certain layers in the Temeside group where they are found in 

 cross-bedded sandstones, in bone-beds, and characteristically, in olive- 

 colored shales. 



