176 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 6, 



Silurian forms), 9 at least range into the Lower Silurian rocks of 

 Britain, whilst 14 is the number, if the Lower Silurian typeof Northern 

 Europe be included. On the plain fact therefore, that there are many- 

 species of Trilobites, Orthidas, and other shells which unite the two 

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

 sj^stem 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 

 Orthidae, 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 shown to abound in Cystidea, those simple forms 

 which, chiefly by the labours of Von Buch, have been shown 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 

 lavss 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 Britam, but 

 the researches of the Government Geological Surveyors detected the 

 common species of Northern Europe {Echino-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 detecting 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, us just announced by Professor Sedg- 

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

 though it was on very general observation, must be abandoned*, I natu- 

 rally rejoice inthisunexpected additional evidence, whereby the Upper 

 and Lower Silurian rocks are still more firmly united in one system \. 



Whilst we are considering what are the natural distinctions of 

 a system, or '* terrain " in the sense in which I used the word, I 



* See Russia in Europe and the Ural Mountains. 



t I also learn from Professor E. Forbes and the geologists of the Government 

 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 Wen- 

 lock shale, and they had been previously observed by the Rev. C. Brodie in the 

 Vv enlock limestone. Although the species of North Wales are not yet described, 

 it is rather remarkable that the Onchus of these Lower Silurian rocks is said to 

 resemble, to a great extent, the 0. Murchisoni, Agassiz, of my Ludlow rocks. 



