The beds with Dictyograptus flabelliformis form a wonderfully constant horizon at or near the top of the Olenus beds. They are found in North Wales, the Border Counties between Wales and England, France, Scandinavia, Russia and Canada.
The passage fauna of the beds which are the equivalents of the Tremadoc Slates may be spoken of as the Ceratopyge fauna, for Ceratopyge forficula, a remarkable species of trilobite, characterises it in Scandinavia, and will probably be found elsewhere. Ceratopyge beds have been found in North and South Wales, Shropshire, Scandinavia, Bavaria and North America, and in each case the fauna is intermediate in character between that of the Cambrian and that of the Ordovician system, containing the loosely-formed trilobites of the former with the more compact ones of the latter. The genus Bryograptus, a many-branched graptolite, also appears to characterise this fauna[73].
[73] For accounts of the Tremadoc Slates Fauna in England and Wales see Ramsay, A. C., Geology of North Wales, Appendix; Hicks, H., Quart. Journ. Geol. Soc., vol. XXIX. p. 39; Callaway, C., ibid. vol. XXXIII. p. 652, whilst many of the foreign fossils are noticed in Brögger's Die Silurischen Etagen 2 und 3 and Barrande's Faune silurienne des Environs de Hof en Bavière.
The faunas of the Cambrian rocks have not been studied in sufficient detail, with reference to the physical surroundings of the organisms, to throw much light upon the conditions under which the strata were deposited, though the evidence obtained from an examination of the lithological characters of the deposits is generally corroborated by study of the organic contents.
THE ORDOVICIAN SYSTEM.
Classification. The Ordovician strata were originally divided into series by Sedgwick as follows:—
Upper Bala,
Middle Bala,
Lower Bala,
Arenig.