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]. [Footnote 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. CHAPTER XV. THE ORDOVICIAN SYSTEM. _Classification._ The Ordovician strata were originally divided into series by Sedgwick as follows:-- Upper Bala, Middle Bala, Lower Bala, Arenig. The Arenig series was at one time included by some writers with the Lower Bala under the name Llandeilo, but the word Llandeilo is now used in the sense of Sedgwick's Lower Bala. The Middle Bala is often spoken of as Caradoc, but the terms Bala and Caradoc are sometimes used interchangeably. As much confusion attaches to the use of the name Bala without explanation, the alternative titles have been largely adopted, and as the series are well defined there is no objection to their use, save that some expression is wanted equivalent to Upper Bala. The local term Ashgill shales was originally applied by Mr W. Talbot Aveline to beds of this age in Lakeland, and I have elsewhere suggested the use of this name for the whole series in that region; its use may well be extended to the series which is developed in many parts of Britain and the continent. The terms which will be used here, therefore, for the different series of the Ordovician system are the following:-- Ashgill Series (= Upper Bala) Caradoc " (= Middle " ) Llandeilo " (= Lower " ) Arenig "