Chap. XI.] 
OLD EED OF THE ' SILUEIAN BEGIOJNV 
245 
ally of great thickness. They also contain the greatest quantity of those irre- 
gular courses of mottled red and green earthy limestones termed 'Cornstone,' 
and which, though usually consisting of small concretionary lumps only, 
expand here and there into large subcrystalline masses, particularly on 
the western face of the Brown Clee Hills, Shropshire. A higher member 
of the series is composed of grey, red, chocolate, and yellow- coloured, fine- 
grained, micaceous sandstones and flagstones, e, which, in certain tracts, 
are surmounted by pebbly beds and conglomerates, /, as along the encir- 
cling underlying edge of the Great South- Welsh Coal-basin. These last- 
mentioned rocks are as well exposed in the lofty escarpments of the Pans 
of Carmarthen and Brecon above alluded to, as in the range of the hills 
near Abergavenny on the south bank of the River Usk. Similar hard 
sandstones and conglomerates constitute, in like manner, a symmetrical 
girdle around the coal-basin of the Forest of Dean, Gloucestershire (see 
Map), where, and in the gorges of the Wye, as well as in the adjacent and 
much larger basin of South Wales, they are everywhere conformably over- 
lain by the shale and limestone which, in these tracts at least, compose 
the base of the Carboniferous rocks. Occasionally, as under the Carboni- 
ferous escarpment of the Clee Hills, Salop, these upper beds are of yellow 
colours. In short, the above generalized section gives an idea of the 
prevalent distribution of the strata within, and wrapping round, the original 
Siluria. 
Now, whilst in all that region the only organic remains which have 
been found in this great red and green series, with the single exception of 
the Crustacean figured on the next page, are fragments of fossil Fishes 
and a few Plants, we shall presently see that, with mineral conditions to 
a great extent similar, the widely spread Old Bed Sandstone of Scotland, 
which is of equivalent age, has also yielded abundance of Ichthyolites. 
This prevalence of Fishes is in striking contrast to all that has been said 
of the great mass of the contiguous and underlying Silurian rocks, in 
which no trace of a vertebrated creature has been found, except in the 
highest member of the system, which ushers in the natural group under 
consideration. 
In Herefordshire and Brecknockshire, as well as in Shropshire, Ichthy- 
olites were formerly described as occurring chiefly in strata where calca- 
reous matter is most diffused, — teeth, bones, and scales of Fishes having 
been often detected in the cornstones, though the best specimens have 
been procured in the finely laminated flagstones and marls which are in 
the vicinity of those concretions. The Fishes of this region, which were 
first made known to geologists by the publication of the ' Silurian System,' 
belong to the genera named (by Agassiz) Cephalaspis, Onchus, and Ptycha- 
canthus; and figures of them, from that work, are here repeated in 
Plates XXXVI. & XXXVII. To these the Pteraspides formerly supposed 
to belong to Cephalaspis (Pteraspis Lloydii and Pt. rostratus, figured in the 
