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52 BRITISH FOSSIL CRUSTACEA. 



Section of Strata below the base of the Old Red Sandstone in the 



District of Lesmahagow. 



/ a. Coarse conglomerate, chiefly of well-rounded fragments of liver-coloured quartz-rock. This is 

 a very persistent band through the whole district. 



b. Red mudstones and shales, with ripple-marks and sun-cracks. 60 to 70 feet. 



c. Grey and green flaggy shales, with hard stone-partings, like portions of the Silurian series. 

 140 feet. 



d. Red mudstones and shales, like those (b) below the quartzite conglomerate. 500 feet. 



e. Red sandstones and fine quartz-conglomerates. 300 feet. 



/. Sandy, flaggy, green stone-bands and shales, with partings of grey and red mudstone passing 

 conformably upwards into e. About 130 feet. 



g. Blue, grey, and green shales, sandy mudstones and sandstone bands, becoming more sandy 

 towards the top, and more shaly and flaggy towards the bottom. 200 feet. These are the 

 Trochus beds ; they contain Platyschisma helicites, three species of Lingula, Beyrichia, and 

 some undetermined shells. 



h. Hard blue and grey flaggy shales, with occasional bands of calcareous nodules. 350 feet. 

 These are the Pterygotus beds. 



i. Hard grey flagstones and bands of hard greywacke. About 500 feet. 



k. Grey, blue, and olive shales, becoming more and more interbedded with hard stone-bands 

 towards the base. About 300 feet. In some of these beds Beyrichia is very abundant ; 

 others contain Ceratiocaris, two or three species of Lingula, some shells not yet determined, 

 Platyschisma helicites, &c. 



I. Hard bands of greywacke, with shale partings. These form the lowest portion of the Silurian 

 series visible in this district. They must be at least 2000 feet thick. Fossils are scarce, but 

 an Orthoceras occurs along with plant-like markings and Ceratiocaris. 



The total thickness of Upper Silurian strata in the neighbourhood of Lesmahago cannot 

 be less than 3500 feet, and, as the base of the series is nowhere seen, the depth may be much 

 more. 



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Although the Pterygoti occurring in the dark clay-slates of the Upper Silurian of 

 Logan Water and the Nethan River are far inferior in size to the gigantic Pt. anglicus 

 from the Lower Old Red Sandstone of Forfarshire and Perthshire, yet for wonderful perfec- 

 tion of preservation they can scarcely be rivalled, and certainly not surpassed, by similar 

 remains from any other formation. Since the preceding description of Pt. anglicus was 

 written, however, I have had the good fortune to visit the Museum of the Natural History 

 Society at Montrose (September, 1867), and inspect the magnificent specimen of Pterygotus 

 anglicus there preserved, from Lord Panmure's Quarries at Carmyllie. This unique 

 example measures about 3 feet 6 inches in length, 13 inches across the widest segment, 

 and is by far the most perfect remain of this species yet met with, having the head united 



