Silurian Bocks of North Wales, ^c. 159 



Europe bo included. On the plain fact, therefore; that there are 

 many species of Trilobites, Orthidsc, and other shells which unite the 

 two groups, I maintain that the Lower Silurian cannot be viewed as 

 a system independent of the Upper. 



But it is not on the duration or passage of species from the one 

 group to the other, that I alone depend for the conservation of the 

 zoological unity of my system. The qualifications and character of 

 what I term a system are chiefly based on the assemblage of its 

 classes of animals. Thus, the Silurian was typified as the great 

 system of Trilobites, which crustaceans rapidly dwindle away in the 

 overlying Devonian, and expire in the Carboniferous system. Again, 

 the Silurian system was represented as being the chief centre of 

 Orthida), its lower half being specially marked by small species of 

 that genus with simple plaits. It was further spoken of as charged 

 with Graptolites, and also as being the horizon in which certain very 

 peculiar chambered shells are most rife. Of late years its lower 

 strata have been shewn to abound in Cystidea, those simple forms 

 which, chiefly by the labours of Von Buch, have been shewn to be 

 the earliest created forms of the great family of Crinoids. And here 

 I would beg British geologists to attend to the importance of foreign 

 comparisons, if they wish to see rock systems founded on laws of 

 general distribution of animals. Abounding in the Lower Silurian 

 rocks of Scandinavia and Russia, these Cystidea had not been found 

 by myself in the Lower Silurian rocks of Britain, but the researches 

 of the Government Geological Surveyors detected the common 

 species of Northern Europe (EcJiino-sphcBrites aurantium) in strata 

 actually described and coloured in the map of the Silurian region by 

 myself as Llandeilo flags, whilst the same observers are now detect- 

 ing the same in greater quantity in the rocks of Bala and in Ireland. 



Even whilst I write, I learn that the only strong distinction which 

 was thought to exist between the Upper and Lower Silurian rocks 

 has vanished by the discovery of the defences of cartilaginous fishes 

 of the genus Onchus in the latter, as just announced by Professor 

 Sedgwick ; and thus, whilst my view of a period void of vertebrata, 

 founded though it was on very general observation, must be aban- 

 doned,* I naturally rejoice in this unexpected additional evidence, 

 whereby the Upper and Lower Silurian rocks are still more firmly 

 united in one system.f 



* See Russia in Europe, and the Ural Mountains. 



t I also learn from Professor E. Forbes, and the geologists of the Govern- 

 ment Survey, that they have detected the defence of an Onchus in the limestone 

 near Bala. Professor Sedgwick states, that the species he mentions were found 

 in the Upper Llandeilo flags. Professor Phillips has detected fish-remains in 

 the Wenlock shale, and they had been previously observed by the Rev. C. 

 lirodie in the AVenlock limestone. Although the species of North Wales are 

 not yet described, it is rather remarkable that the Onchus of these Lower Silu- 

 rian rocks is said to resemble, to a great extent, the 0. Murchisoni, Agassiz, of 

 my Ludlow rocks. 



