The trilobites reach their zenith of development and exhibit great variety in form, but whilst Ceratopyge is found only at the base, and forms like Chasmops and Phillipsinella only in certain beds in the Upper Ordovician, many genera or families have a long range ; Ogygia for example is characteristic of the Lower Ordovician when it occurs, but the nearly allied Asaphus is found at all horizons. The forms of commonest occurrence are Asaphus, Ogygia Acaste, Calymene, Placoparia, Illaenus, Homolanotus, Remopleurides, Cheirurus, Lichas, and Phillipsinella; there are also blind trilobites such as Trinucleus and Ampyx, both of which are common, as is also Aeglina with its over-developed eyes.
Various Orthoceratidae are abundantly represented in the Orthoceras Limestone of Scandinavia; taken as a whole however the mollusca (q.v.) play a far more important part in the fauna of America than they do in Europe, both gasteropods and lamelli branchs being abundant at certain horizons, whilst the bryozoa are also important in America, and probably on account of the physical conditions, are far more abundantly represented.
Distribution.—Ordovician rocks have an almost world-wide distribution since they have been recorded from both Arctic and Antarctic regions, Europe, North America, South America (Bo livia), Asia (China, Burma, northern Himalayas, Shan States), northern Africa and Australia. The classic areas of development however are those of Europe and North America, and the other areas may be regarded in terms of these. In Europe they may be considered as having been laid down in two main areas, a northern area and a southern area. The northern area of deposition com prised the British Isles, the greater part of Scandinavia and the Baltic Provinces ; it consisted of two gulfs penetrating far into the north Atlantic continent, the outermost of which was probably a true geosyncline and the inner an epicontinental sea; these were partially separated by a long island or peninsula extending from Scandinavia to Britain. The southern area is considered to have lain to the south of a land mass extending through Central Europe and comprising southern France, Spain and Portugal, Thuringia, the Fichtelgebirge, Kellerwald and Bohemia.
The differences in the faunas of these two areas has been re garded as in large measure dependent upon the existence of the separating land barrier, but it is possible that these differences may be due in part to facies, and the possibility of the effect of migra tion and counter-migration along the shore line must not be over looked. Towards the close of the period at any rate this difference in faunas disappeared, for a uniform type of fauna characterises all areas of deposition whether they belong to the shallow-water or deeper-water facies.
In America shallow water facies of varied type predominated all along the submerged margin of the Canadian shield (epicontinental sea), but in the east and west, definite geosynclines originated, known as the Cordilleran and Appalachian geosynclines respec tively, and it is to these that the Graptolite Shale facies mainly belong.
British Isles.—The British Ordovician rocks have always been much folded since their formation. They were laid down originally within the geosyncline of the northern area of deposition, part of the northern margin of which appears to have lain along the High land border, whilst its southern shore-line ran obliquely through the Welsh borderland and thence south into Cornwall. Hence in the neighbourhood of the Highland border (Girvan) and the Welsh borderland there is an extensive development of the shal low-water facies, which traced south-east on the one hand and more generally west and north-west on the other, pass into de posits of the deeper water facies characterised by graptolites. These sediments are however interrupted at different horizons by volcanic rocks though the intensity and duration of vulcanicity varied in different places. It is the folding of the sediments and the occurrence of these volcanic rocks on a grand scale that is re sponsible for the rugged mountain scenery of both north Wales and the Lake District. There appear in general to have been two main volcanic episodes, an earlier one (Arenig) characteristic more particularly of the marginal areas, of the geosyncline, and a later one (Llandilo-Caradoc) belonging to its more central portions. The table shows the succession in the classic areas of development.
European Continent.—In Scandinavia the Ordovician rocks have escaped the severe tectonic disturbances that have so gen erally affected the rocks of this age in other regions. In Norway their main development occurs in the Christiania (Oslo) basin, but in Sweden the best sections are those seen in the many table mountains that have been cut out by circumdenudation, where the rocks can be studied lying horizontally one above the other. The recognition of the different facies of deposition is here par ticularly important ; the classic areas are those of Sickle, bland, ostergotland, Vastergotland and Dalarne, and in these the shallow water and deeper water facies show varying degrees of develop ment. In Rine is found the most complete development of the Graptolite Shale facies, but this is associated in places with some Trilobite Shales and some thin bands of Orthoceras Limestone. In Dalarne on the other hand the facies is largely shallow water throughout and almost entirely calcareous, a similar facies being found in Oland where however the succession is incomplete above the horizon of the Cystid Limestone, and Dictyograptus Shales largely replace the Ceratopyge Limestone ; in Ostergotland the Ceratopyge Limestone is again replaced by Dictyograptus Shales but the upper part of the succession is complete, and in Vaster gotland Graptolite Shales not only replace the Ceratopyge Lime stone but also the lower part of the Orthoceras Limestone. Hence the general succession in the different areas may be regarded as dependent upon the degree to which the various limestones are replaced by Graptolite Shale.