24 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 21. 



In North America it has been long known, from the writings of 

 Dekay and Harlan, that large Eurypteri occur in a so-called black 

 greywacke slate at Westmoreland in Oneida County, New York, 

 which will probably be found to be on the parallel of the Upper 

 Ludlow Rock. The discovery of the large Burypteridce in the same 

 zone as at Lesmahago in other regions is therefore peculiarly satis- 

 factory. It is however to be observed, that in tracts far removed 

 from each other, different, though closely allied, species make their 

 appearance. Thus, whilst the Pterygotus is perhaps the usual and 

 most characteristic type, the species found in Scotland* is said to be 

 different from that known in the Silurian region. Near Kendal in 

 Westmoreland the genus Eurypterus occurs with Pterygotus ; whilst 

 in Russia the former seems to be the prevalent genus. 



In the Lanarkshire case, Mr. Salter finds the same sculptured 

 plates which have been usually referred to the Pterygotus, and also 

 the small Lingula cornea and Trochus helicites of the Uppermost 

 Ludlow Rock. With these he has also detected in the rich collection 

 of Mr. Slimon five or six new forms of a large crustacean which he 

 terms Himantopterus, and describes in the following memoir, p. 26. 

 With them too, another genus, the Leptocheles of M'Coyf, has been 

 found, which the fine specimens now collected show to be simply the 

 caudal portion of the Ceratiocaris, a genus which is found as low 

 as the Wenlock Limestone. And yet, with these distinctions of vary- 

 ing forms, which are everywhere recognizable in the Silurian zones of 

 similar age in distinct regions, we find this group of animals con- 

 sistently and uniformly defining the same zone of sediment over the 

 Northern hemisphere. 



Wherever these large Crustaceans are found, and with them small 

 Lingulce and other fossils, we may be sure that we are at or near the 

 very summit of all rocks to which the term Silurian can be applied, 

 and that the next overlying stratum belongs to the first great sera of 

 fishes, the Devonian or Old Red Sandstone ; for the thin transition- 

 band now under consideration still remains what I stated it to be 

 twenty-one years ago, the lowest in which the trace of a true verte- 

 brated animal has been detected. 



In Scotland, where we had despaired of finding any representative 

 of the Ludlow formation, the discovery of Mr. Slimon is indeed highly 

 gratifying. Perforated as are the lower edges of the coal-basins which 

 occur along the northern frontier of the Silurian rocks of the South of 

 Scotland, by various igneous rocks which have to a great extent up- 

 heaved the Old Red Sandstone, we may, after this discovery, look to 

 the detection of other links to connect the Orthoceratite-rocks of the 

 Pentland Hills with the shelly deposits of Girvan and the younger 

 strata of Lesmahago, and thus evolve a full series of both Lower and 

 Upper Silurian Rocks in North Britain, where their very existence 

 was until recently almost ignored. 



* I have directed that these remarkable Crustaceans, as well as others of the 

 same age in Herefordshire, be figured and fully described in a Decade of the Me- 

 moirs of the Geological Survey of Britain. — R. I. M., January 1856. 



t See Pal. Foss. Cambr. Mus. Pl.I E.fig.7; Quart. Journ. Geol. Soc. vol. ix.p.13; 

 and also a full recognition of Prof. M'Coy's abihty in separating some of these 

 crustaceans from fishes with which they had been confounded, in " Siluria," p. 236. 



