140 THE HABITAT OF THE EURYPTEEIDA 



localities throughout the belt east of Langholm, showing the basal 

 grits and greywackes projecting up through the second division of 

 the Wenlock. This distribution indicates the presence of the basal 

 sandstone of the advancing sea throughout the southern belt. The 

 single eurypterid fragment found in this belt, it will be recalled, was 

 discovered in the track-crowded shales and greywackes at Slitrig 

 Water. These being interpreted as basal beds of an advancing sea, 

 it is most natural to expect that the sea, rolling landwards and up 

 the rivers, slowly but unceasingly converting the dry land into sea- 

 floor, should catch river- dwellers who were not able to or did not 

 migrate upstream fast enough, and even if there were none such, at 

 least dead remains would inevitably be passed over by the sea in 

 its continued advance. One would undoubtedly expect more than 

 a single fragment and probably more will be found in the southern 

 Wenlock rocks. The more abundant occurrence in the Pentland 

 Hills is explainable on the supposition that the sandy bands contain- 

 ing the broken exoskeletons represent the outwash from rivers into 

 the sea, of shed exoskeletons and maybe even of the remains of euryp- 

 terids which were killed off in great numbers by the entrance of salt 

 water into the rivers. So soon as this group of organisms was able 

 to migrate far enough away from the sea which had overtaken the 

 earlier individuals, the appearance of exoskeletons in that region 

 would come to an end, but one would expect similar catastrophes to 

 occur in another locality at a higher horizon. Unfortunately, the 

 exact method of entombment must remain hypothetical, since the ex- 

 posures are so few, but that the eurypterids did not live in the Wen- 

 lock sea is apparent. One further argument which might be adduced 

 is that in the purely marine, open-sea Wenlock of England, not a trace 

 of a eurypterid has been found, although if they were true marine 

 organisms during Lower Siluric time as most geologists claim, then 

 it is surprising that they alone of the marine fauna should be found 

 only in southern Scotland although migration was open along most 

 convenient marine channels into Wales. 



8. UPPER SILURIC OF OESEL 



For beauty and perfection of preservation no other known euryp- 

 terid remains can compare with those from the island of Oesel. 

 Though only five species have been found and only one in abun- 

 dance, the lack of a varied fauna is entirely compensated for by the 



