In Silurian and Devonian Epochs 103 



ed with carbonaceous fragments, but in which bones are 

 rare. Purple-red sandstones with shaly partings come on 

 immediately above this 'Fragment-bed;' these differ in gen- 

 eral lithology from anything that we have seen at a lower 

 horizon; and since the 'Fragment-bed' at their base appears 

 fairly constant over wide areas and is easily recognizable, 

 we suggest its adoption as the upper limit of the Silurian 

 system in this district." 



To complete the evidence required, their tables giving 

 the palaeontological distribution of all known species are 

 appended to the paper. 



Now an inspection of this extensive table (pp. 219-20) 

 brings out very clearly that exactly the difference in fauna 

 of the freshwater Old Red rocks and the marine Devonian 

 rocks, observed in the next formation to be studied, applies 

 also here. For on p. 219 and the upper half of p. 220, the 

 organisms are all typical marine invertebrates which occur 

 below the Ludlow Bone-bed, if we except rare occurrences 

 of some brachiopod shells, which evidently represent a 

 temporary sea invasion or inwash, when the "soft-mud- 

 stone" was deposited (p. 203) between the Lower and 

 Upper of the two Ludlow beds. The organisms listed how- 

 ever on the lower half of p. 220 under "Crustacea" and 

 "Pisces" are freshwater, and attain their climax of abund- 

 ance alike in species and individuals in the Upper or Teme- 

 side Bone-bed and associated Olive shale. Here Pachytheca, 

 Eurypterids, Leperditia, Physocaris^ and abundant fishes 

 form a characteristic freshwater companionship, such as is 

 carried forward in facies into the Old Red rocks. The 

 intrusion here of a few Lingula shells can be quite readily 

 explained in one of at least four ways. But at present we 

 reserve discussion of this. 



Observations made by King and Lewis (59:437) in 

 the South Staffordshire region closely parallel those just 

 given 



On the east side of Scotland in the Pentland Hill region, 

 and on the west in the Leshmahagow region, members of 

 the Geological Survey have mapped Silurian areas that seem 

 exactly to correspond with the beds already described, and 



