114 
SILURIA. 
[Chap. VI. 
oil the banks of the Severn, near Coalbrook Dale and the Iron Bridge, 
where it is called ' Die Earth and is thence to be followed all along the 
escarpment of Wenlock Edge, occupying a broad valley of denudation, 
called Apes Dale, between that ridge and the Caradoc. In the Malvern , 
and Woolhope districts it is also a mass of finely levigated argillaceous 
matter, the lower part of which is more calcareous than in Shropshire, and 
in parts sandy and gritty. Near Malvern, where the shale is in parts 
highly charged with fossils, particularly the small Brachiopoda figured in 
PL XXII., Professor Phillips estimates its thickness at about 640 feet. In 
some parts of Wales the Wenlock shale is as incoherent as in the adjacent 
English counties ; but in Denbighshire, as before said, it is represented by 
hard slaty sandstones and schists. 
The prevailing fossils of the Wenlock shale, exclusive of Trilobites and 
Corals, which are chiefly species unknown in the lower deposits, are 
Brachiopods of the genera Leptsena, Orthis, Strophomena, Atrypa, and 
Rhynchonella, mostly of small size. In a general way the fossils of this 
stratum, the chief of which are given in Plates XX. and XXII., are the same 
as those of the overlying limestone. Among them, however, are several 
common to this deposit and the Lower Silurian rocks, such as Orthis ele- 
gantula (so abundant in the slates of Snowdon), Strophomena pecten, S. 
depressa, Atrypa marginalis, A. reticularis, and Spirifer plicatellus. 
Most of these Lower Silurian forms have indeed a much greater vertical 
range, and pass upwards through the Wenlock high into the Ludlow 
formation. 
Orthis biloba of Linnaeus, 0. hybrida, and the large, flat Orthis rustica are 
characteristic shells, as well as Leptaena laevigata, L. transversalis, Pentamerus 
linguifer, Athyris tumida, Rhynchonella rotundata, R. depressa, R. Stricklandi, 
R. deflexa, and R. sphaerica, and, in more western tracts, R. navicula. 
Aviculse, Ctenodontae, and some other bivalve Shells occur frequently ; but 
few of them are characteristic except the Cardiola interrupta. Of spiral Shells, 
Euomphalus funatus and E. alatus, Pileopsis haliotis, Turbo (?) cirrhosus, Bel- 
lerophon Wenlockensis, with B. dilatatus, are the most common. The Pteropod 
Mollusks Theca Forbesi and T. anceps are not scarce. Certain Orthocerata, 
such as Orthoceras annulatum, 0. filosum, and 0. angulatum, are rare ; but many 
of the thin-shelled species, O. subundulatum, 0. primaevum, and others abound 
in these muddy sediments. They are almost the only shells in this formation 
as it is exhibited in Denbighshire and other parts of North Wales, and occur 
there in the greatest abundance. Phragmoceras of one or two species, and cer- 
tain Lituites, e. g. L. articulatus, L. Biddulphi, and occasionally L. giganteus, 
are conspicuous forms ; but the two last-mentioned genera are not commonly 
met with. 
Of Trilobites, Encrinurus punctatus and E. variolaris, Calymene Blumenbachii 
and C. tuberculosa, Phacops caudatus and its variety P. longicaudatus, are cha- 
racteristic ; but one of these, the Calymene Blumenbachii, is known in the rocks of 
Snowdon. The genera Trinucleus, Asaphus, and Ogygia are never detected even 
* So called because this stratum lies beneath all the mining-ground. 
