134 
SILURIA. 
[Chap. VII. 
that the bed was a heap of broken beetles. Dr. Harley has truly re- 
marked* that this bed is often very compact, and of a lighter colour, and 
closely resembles the cake from which linseed-oil has been expressed. 
The supposed Fishes of this stratum, as exhibited in my original work, must 
now, it seems, be reduced in number. At all events, besides the remarkable 
Pterygotus, portions of which are figured in PI. XIX. f. 4-6, and which was re- 
moved by Agassiz himself to the class of Crustaceans, Professor M'Coy has 
diminished the list of Ichthyolite remains by proving that some of the supposed 
fish-defences should also be removed to that group. One of these, to which he 
has applied the name Leptocheles Murchisoni (see PL XIX. f. 1, 2), was formerly 
figured as an Onchus, or fish-defence f. But although these may be removed from 
our scanty list of Upper Silurian vertebrata, the ichthyic nature of several speci- 
mens figured in PI. XXXV. figs 1-18, is evident from their external characters, 
and has been proved satisfactorily by the microscopic researches of Dr. Harley, 
who has also discriminated numerous minute Crustacean organisms, including 
Conodonts, in the Bone-bed. (See Chap. X.) 
The capping of the Bone-bed is composed of light-coloured, thin-bedded, 
slightly micaceous sandstones, in which quarries are opened out near Downton 
Castle on the Teme (the Downton-Castle stone, Sil. Syst. p. 197). It is trace- 
able also in the vicinity of Malvern. 
The uppermost layers of the whole Silurian system, and which form a 
transition into the Old Bed Sandstone, consist of thin-bedded flagstones, 
occasionally reddish, and in certain districts forming ' Tilestones : ' these 
contain Lingula cornea (PI. XXXIV. f. 2, a fossil found in the Ludlow 
rock), with Crustaceans, fin-rays of Pishes, and remains of the singular 
fish Pteraspis (PL XXXVI. f. 9, 10, 11). 
Being compelled in my earlier researches to draw a line of demarcation 
between the Upper Ludlow formation and the bottom of the overlying Old 
Bed Sandstone, I then included the tilestones in the latter, — particularly 
as in most parts of the region a portion of them decomposes into a red 
soil, thus affording a clear physical line of demarcation between them and 
the inferior grey rocks $ . The fossils which were then figured as character- 
istic of such tilestones exhibited little else than species common to the 
Upper Ludlow rock, and were chiefly obtained from this formation as it 
ranges through Clun Forest and some parts of South "Wales, where the 
Bone-bed has not yet been seen. These species and others since discovered 
have, indeed, for seventeen years led me to classify these tilestones with 
the Silurian rocks, of which they form the natural summit. For, in their 
* Quart. Journ. Geol. Soc. vol. xvii. p. 543. maps. Geologists who have had to labour with 
t For further illustration of this point, see imperfect topographical assistance in a region 
Quart. J ourn. Geol. Soc. Lond. vol. ix. p. 12 ; and which, like this, had been wholly unexplored by 
vol. xvii. p. 545. miners, are not those who will criticise errors of 
I The reader who may refer to my original detail, which have been remedied by those who 
map must recollect that it was constructed be- followed me. The classification and chief outlines 
tween the years 1831 and 1836. At that time I of my map and sections, as far as they relate to 
coloured such sheets of the Trigonometrical Survey my own Silurian rocks, have indeed, in all essen- 
as had been published; and from those and other tial points, been sanctioned by the Geological 
rude materials my map was constructed. In fact Survey and the numerous geologists, led by Pro- 
nearly all the country of Wales to the west of my fessor Phillips, who have succeeded me. 
typical region was then undefined upon accurate 
