126 
SILURIA. 
[Chap. VII. 
fuller's earth (provincially ' Walker's earth ') beneath heavy masses of the 
limestone, which rest upon it, which has occasioned numerous landslips 
both near Ludlow and in other parts of Herefordshire, one of the most 
striking of which will presently be mentioned. 
The lower shale occupies the escarpments and contiguous valleys of the Lud- 
low rocks which range from Shropshire by Presteign to Radnor Forest, and also 
large undulating tracts of the western parts of Shropshire or contiguous parts ox 
Montgomeryshire, such as the Long Mountain and other tracts around Welsh- 
pool and Montgomery (see Map). From the steeps and valleys west of King- 
ton, it extends, together with the upper members of the Ludlow rocks, to the 
banks of the Wye, and is finely exposed in the noble escarpment at the western 
end of the Forest of Mynydd Epynt in Brecknockshire. A good idea of the fea- 
tures of that tract is conveyed by a sketch already given, p. 58, where the 
rounded outline of the hills in the middle ground, as seen from the slaty hills on 
the west, is due to the soft nature of this deposit. The same rock is also well 
developed in the Malvern tract, where Professor Phillips assigns to it a thickness 
of about 750 feet. (See also section, p. 95.) 
In the W T oolhope elevation and the group of Usk, or as lying between the 
Dudley and Sedgeley (Wenlock and Aymestry) limestones in Staffordshire, 
it is everywhere the same dull, non-micaceous shale, which, from its incoherence, 
has been denuded for the most part, thus giving rise to a deep valley which 
separates the harder parts of the Wenlock and Ludlow rocks from each 
other. 
In the environs of Ludlow, and in many parts of the Silurian region, 
this inferior member of the Ludlow rocks, whilst containing several 
forms of Trilobites found also in the strata above and below it, is specially 
characterized by a profusion of straight or curved Chambered Shells. Such 
are the Orthoceras, Lituites, and Phragmoceras, a genus named by my old 
friend the late W. Broderip, the eminent naturalist, and which was un- 
known before the publication of the ' Silurian System.' Orthocerata abound, 
not less than eleven species having been figured as characteristic of this 
rock. But extended researches have shown that, in this case as in many 
instances of other fossils alluded to, several of these Chambered Shells occur 
in much older as well as younger members of the system. 
In the rich locality already spoken of, viz. Church Hill, Leintwardine, 
the usual fossils of this rock have been found associated with Starfishes and 
that singular Crustacean the Pterygotus, together with some other forms, 
and among them a fragment of the Fish Pteraspis. This last discovery has 
of course modified my former belief, that the fishes in the upper part of the 
Ludlow formation were the oldest known ichthyolites. But, after all, the 
modification is slight ; for still the position of this Pteraspis is scarcely 
beneath the real centre of the Ludlow formation as a whole. 
The Pterygotus belongs to a distinct species from that found in other localities, 
and has been described by Mr. Salter under the name of P. punctatus. It was 
apparently of great size, perhaps seven or eight feet in length ; but its fragments 
