The geologic record is, as perhaps is to be expected, exceedingly poor, except as regards the calcareous Siphonales, which are well represented at various horizons, from the Silurian to the Tertiary; even the Diatomaceae, which are found in great quantities in the Tertiary deposits, do not occur at all earlier than the chalk.
Archaean granite is thus exposed at Yampol and other places in Russia, and this is followed towards the west by Silurian and Devonian beds in regular succession - the Devonian being of the Old Red Sandstone type characteristic of the British Isles and of Northern Russia.
They rest unconformably on the Silurian rocks on the King river and to the west are faulted against the schists by a powerful overthrust fault, traversing the Mount Lyell copper field.
The northern periphery lies in French Congo: the western boundary is formed by a zone of Archean and metamorphic rocks and a region composed of several rock groups considered to range between the Silurian and Carboniferous periods; but it is only in the limestones of one group that fossils, indicating a Devonian age, have been found.
Overlying these amongst the Palaeozoic rocks, we meet in many parts of Japan with slates and other rocks possibly of Cambrian or Silurian age.
The Phyllopoda, Ostracoda and Cirripedia (Thyrostraca) are represented in Cambrian or Silurian rocks by forms which seem to have resembled closely those now existing, so that palaeontology can have little light to throw on the mode of origin of these groups.
The evidence for terrestrial Silurian vegetation is still dubious; apart from some obscure North American specimens, the true nature of which is not established, Potonie has described well-characterized Pteridophytes (such as the fern-like Sphenopteridium and Bothrodendron among Lycopods) from supposed Silurian strata in North Germany; the horizon, however, appears to be open to much doubt, and the specimens agree so nearly with some from the Lower Carboniferous as to render their Silurian age difficult of credence.
Modiolopsidae.-Extinct; Silurian to Cretaceous; adductor muscles sub-equal.
Storriei, from the Silurian (Wenlock) of South Wales, described by Barber, there is no sharp differentiation of the two kinds of tubes; they are rarely observed to branch, except in the gaps, which in this species are not radially directed.
The Silurian scorpion Palaeophonus, differs, so far as obvious points are concerned, from a modern scorpion only in the thickness of its legs and in their terminating in strong spike-like joints, instead of being slight and provided with a pair of terminal claws.