208 Murchison s Silurian System. 



the upper, cream-coloured, with a splintery conchoidal frac- 

 ture and dull lustre ; the lower, though essentially the same, 

 is cellular, the cavities filled with crystallized carbonate of lime 

 and black bitumen, and veins of calc spar and sulphuret of iron 

 are also disseminated through it. " On first examination," 

 says Mr. Murchison, " I was struck with the strong resem- 

 blance of this rock to certain lacustrine limestones of France 

 and Germany, which by their imbedded organic remains are 

 known to have been formed under fresh water ; and when 

 I further discovered, in 1832, that nearly all the fossil remains 

 contained in it were of terrestrial and fluviatile origin, I 

 named it fresh water limestone. The characteristic fossil of 

 this limestone is a very minute discoid univalve, resembling 

 on first inspection Planorbis nautilus, Flem., and with this is 

 associated a small bivalve, having the form of a Cyclas, and 

 also a small Cypris. In addition to the above named fossils, 

 the remains of fishes have been discovered, the most re- 

 markable of which is a new species, Ctenodus Murchisonii, 

 Ag. and Megalichthys Hibberti." We shall presently see 

 how these organic remains enabled Mr. Murchison to iden- 

 tify another bed of limestone, in a very different part of the 

 country, as belonging to the same age, and formed under the 

 same circumstances with the one just described. Among 

 the plants of the Shrewsbury coal measures, the most curious 

 is one nearly allied to the grasses, named Cyperites bi- 

 carinata, and which is also found in the culm-bearing strata 

 of Devonshire ; a plant intermediate between Lepidophyllum 

 lanceolatam and L. Najus of the Newcastle coal field, and 

 hence called L. intermedium, it is supposed to be allied to 

 the coniferous genus Podacarpus ; also Lepidastrobus varia- 

 bilis, Lind. and Hut. supposed to be the organ of fructifica- 

 tion of a Lepidodendron, it is also found in Newcastle-on- 

 Tyne coal beds ; a Lycopodites, the same as in Bohemian 

 coal fields ; Neuropteris cordata, Brong. also found in De- 

 vonshire culm-bearing strata, St. Etienne, and Alais in 

 France ; Nuropteris gigantea, common to the culm-bearing 



