344 Proceedings of the Royal Physical Society. 



evidence as the not finding of a fossil form in determining 

 the relative age of a series, I am enabled by the kindness 

 and courtesy of our President, Dr Traquair, to demonstrate 

 that the remains of Pterygoti as colossal as those from the 

 Arbroath beds occur high up among the Caithness flags. 

 The same evidence therefore would rob Sir Roderick of his 

 Middle Old Red Sandstone as far as Caithness is concerned. 

 On other and more satisfactory evidence Dr Archibald Geikie 

 has shown that there is really no means of separating the 

 two groups in Caithness, both of which he classes with the 

 lowest division of the Old Red.* He further shows that there 

 is every reason to believe that the Caithness beds were in 

 the main the equivalents of those of Forfarshire, though they 

 were accumulated in an area which was cut off from the 

 more southern one by the barrier of old rocks of the 

 Grampians, — in fact that the Low^er Old Red Sandstone in 

 Scotland was a lacustrine formation, and that the two sets of 

 strata were in all probability deposited almost simultaneously 

 in different lake basins. 



In 1858 a fragment of what appeared to be an animal 

 allied to Pterygotus was exhibited at the British Association 

 Meeting at Aberdeen, which had been found in the island of 

 South Ronaldshay. The rocks in this island are the con- 

 tinuation of the highest beds of the Caithness riagstones.-(- 



A short time ago Dr Traquair drew my attention to some 

 Eurypterid remains among a lot of fossils collected from the 

 Caithness Flagstones in the neighbourhood of Thurso by the 

 late John Miller, and acquired from his heirs for the Museum 

 of Science and Art, Edinburgh. Some of the specimens bear 

 the date 1859. 



Among the fragments collected by John Miller there is at 

 least one specimen allowing of description, and of being 

 named, and which appears to be new. It also throws light 

 on the structure of Pterygotus, while others illustrate the 

 mode of formation of some curious circular quoit- like inipres- 



* "Old lied Sandstone of Western Europe" (Trans. Roy. Soc, Edinb., 

 xxviii., pp. 345-452). 



+ Mr C. W. Peach informs me that this specimen is deposited in the 

 British Museum. 



