J/v i>. N. Peach 071 the Occurrence of Picrygotits, etc. 343 



XXXVI. On the Occin^rence of Pterygotns and a Limuloid in the 

 Caithness Flag stones, and on the Nature and Mode of Forma- 

 tion of' Adam's Plates!' By B. N. Peach, Esq, A.E.S.M., 

 of the Geological Survey of Scotland. [Plate VII.] 



(Read 17th January 1883.) 



Between the years 1860 and 1867, C. W. Peach obtained 

 from Sarclet, in the neighbourhood of Lybster, on the east 

 coast of Caithness, a fragment of a Eurypterid exhibiting a 

 similar sculpturing to that of Pterygotus. The beds from 

 which the specimen was derived are supposed to be the same 

 as those which underlie the Caithness Flagstones at the 

 " Ord," and which, after disappearing under the latter near 

 Berridale, here rise from beneath them in the form of a low 

 arch. These rocks Sir Pioderick Murchison classed with his 

 Lower Old Pted Sandstone, while he looked upon the Caith- 

 ness Flagstones as forming his Middle division. He there- 

 fore attached much importance to the finding of Pterygotus 

 among these Sarclet rocks, as he considered that it confirmed 

 his opinion that these low beds were the equivalents of the 

 Forfarshire flags, thus proving that they were of Low^er Old 

 lied Age. 



In his account of the discovery published in the fourth 

 edition of " Siluria," * however, he makes a singular mistake, 

 for he says, " Since the last edition of this work an important 

 confirmation of my view respecting these lower sandstones 

 being the equivalents of the Arbroath beds or the Lower Old 

 Eed of Scotland has been made by Mr Charles Peach. That 

 successful fossilist detected in strata near Lybster, much 

 older than the Caithness flags, the remains of a portion of 

 Pteraspis — an ichthyolite never yet found above the lowest 

 of the three divisions of the Old Eed Sandstone." f 



To show how little dependence can be put on such negative 



* Pp. 257, 258. 



f In writing the above passage Sir Roderick Murchison must have been 

 trusting entirely to memory, and writing without notes, as he confounds 

 Pterygotus with Pteraspis. There can be no doubt that, at one time, at 

 least, he was aware of tlie nature of the fossil which C. W. Peach handed 

 over to him, and which had been studied by Salter, and pronounced to be u 

 portion of Pterygotus. 



