UPPER SILURIAN GROUP.
Lithological Characters. Thickness. Fossils.
{ Passage Beds, Tile-stones, and } { Sea-weeds,
{ Downton sandstones, at the } 80 { Lingulæ,
{ base of the bone-bed } { Mollusca.
{ {
{ Micaceous, yellowish and } 700 { Crustacea and
Ludlow { grey, sandy mudstone } { Fish-remains.
Rocks { {
{ Argillaceous (Aymestry) limestone } 50 { Crinoids.
{ {
{ Argillaceous Shale with impure } 1000 { Mollusca of
{ limestones } { many genera.
{ Argillaceous or semi-crystalline } { Mollusca of
{ limestone } { many genera.
{ } {
{ Argillaceous shales, in places } { Echinodermata;
Wenlock { slaty } 3000 { Actinozoa;
Rocks { } { Trilobites.
{ Woolhope Limestone and } {
{ occasional bands of argillaceous } { Graptolites.
{ nodules } {
Upper { Grey and yellowish sandstones } { Pentamerus
Llandovery { (occasionally conglomerates) } 800 { oblongus,
Rocks { with bands of } { Rhynchonella,
{ limestone } { Orthides, &c.
Among the fossils of this period may be remarked a number of Trilobites,
which then attained their greatest development. Among others, _Calymene
Blumenbachii_ (Fig. 23), some _Cephalopoda_, and _Brachiopoda_, among
which last may be named _Pentamerus Knightii_, _Orthis_, &c., and some
Corals, as _Halysites catenularius_ (Fig. 26), or the chain coral.
[Illustration: Fig. 23.--Calymene Blumenbachii partially rolled up.]
The Trilobites, we have already said, were able to coil themselves into
a ball, like the wood-louse, doubtless as a means of defence. In Fig.
23, one of these creatures, _Calymene Blumenbachii_, is represented in
that form, coiled upon itself. (See also _Illænus Barriensis_, Fig. 25.)
Crustaceans of a very strange form, and in no respects resembling the
Trilobites, have been met with in the Silurian rocks of England and
America--the _Pterygotus_ (Fig. 27) and the _Eurypterus_, (Fig. 24).
They are supposed to have been the inhabitants of fresh water. They were
called “Seraphim” by the Scotch quarrymen, from the winged form and
feather-like ornamentation upon the thoracic appendage, the part most
usually met with. Agassiz figured them in his work on the ‘Fossil
Fishes of the Old Red Sandstone,’ but, subsequently recognising their
crustacean character, removed them from the Class of Fishes, and placed
them with the _Pœcilipod Crustacea_. The _Eurypteridæ_ and _Pterygoti_
in England almost exclusively belong to the passage beds--the Downton
sandstone and the Upper Ludlow rocks.
[Illustration: Fig. 24.--Eurypterus remipes. Natural size.]
Among the marine plants which have been found in the rocks corresponding
with this sub-period are some species of Algæ, and others belonging to
the Lycopodiaceæ, which become still more abundant in the Old Red
Sandstone and Carboniferous Periods. Fig. 28 represents some examples of
the impressions they have left.
The seas were, evidently, abundantly inhabited at the end of the Upper
Silurian period, for naturalists have examined nearly 1,500 species
belonging to these beds, and the number of British species, classified
and arranged for public inspection in our museums cannot be much short
of that number.
[Illustration: Fig. 25.--Illænus Barriensis.--Dudley, Walsall.]
Towards the close of the Upper Silurian sub-period, the argillaceous
beds pass upwards into more sandy and shore-like deposits, in which the
most ancient known fossil Fishes occur, and then usher us into the first
great ichthyic period of the Old Red Sandstone, or Devonian, so well
marked by its fossil fishes in Britain, Russia, and North America. The
so-called fish-bones have been the subject of considerable doubt.
Between the Upper Ludlow rocks opposite Downton Castle and the next
overlying stratum, there occurs a thin bed of soft earthy shale, and
fine, soft, yellowish greenstone, immediately overlying the Ludlow rock:
just below this a remarkable fish-deposit occurs, called the Ludlow
bone-bed, because the bones of animals are found in this stratum in
great quantities. Old Drayton treats these bones as a great marvel:--
“With strange and sundry tales
Of all their wondrous things; and not the least in Wales,
Of that prodigious spring (him neighbouring as he past),
That little fishes’ bones continually doth cast.”