“ . eS 
676 STRATIGRAPHICAL GEOLOGY. — [Boox VI. 
Among the fossils are some traces of fucoids; sponges (Cliona, a 
burrowing form like the modern Cliona) ; the widely-diffused Monograptus 
(Graptolithus) priodon ; a number of corals (Petraia, Heliolites, Fawosites, 
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| Halysites, Syringopora, &c.); a few crinoids and the earliest known sea- 
urchins (Palzchinus) ; the genus Tentaculites, by some naturalists classed 
with the pteropods, by others with the annelides, is particularly abundant ; 
a number of trilobites, of which Phacops Stokesii, P. Weaveri, Encrinurus 
punctatus, Calymene Blumenbachii, Proétus Stokesii, and Illenus Thomsoni 
are common; numerous brachiopods, as Atrypa hemspherica, A. reticularis, 
Pentamerus oblongus, Stricklandinia lirata, S. lens, Lepteena transversalis, — 
Orthis calligramma, O. elegantula, O. reversa, Strophomena compressa, S. 
pecten, and Lingula parallela; lamellibranchs of the mytiloid genera 
Orthonota, Mytilus, and Modiolopsis, with forms of Pterinea, Ctenodonta, and 
Lyrodesma ; gasteropods, particularly the genera Acroculia, Raphistoma, 
Murchisonia, Pleurotomaria, Cyclonema, Holopella ; heteropods, particularly 
the species Bellerophon dilatatus, B. trilobatus, and B. carinatus; and 
cephalopods, chiefly Orthocerata, with some forms of Actinoceras, Cyrtoceras, 
Tretoceras, and Phragmoceras, and the old species Lituites cornu-arietis. 
2. Wenlock group.—This suite of strata includes the larger part of the 
known Upper Silurian fauna of Britain, as it has yielded no fewer than 168 
genera and 530 species. In the typical Silurian area of Murchison, it 
consists of two limestone bands (Woolhope and Wenlock), separated by 
a thick mass of shale (Wenlock Shale). The following subdivisions in 
ascending order are recognized. 
(a.) Tarrannon Shale-——Above the Upper Llandovery beds comes a 
very persistent zone of fine, smooth, light grey or blue slates, which has 
been traced down the whole length of Wales from the mouth of the 
Conway into Carmarthenshire. These rocks, termed the “ paste-rock” 
by Sedgwick, have an extreme thickness of 1000 to 1500 feet. Poor in 
organic remains, their chief interest lies in the fact that the persistence 
of so thick a band of rock between what were supposed to be continuous 
and conformable formations should have been unrecognized until it was 
proved by the detailed mapping of the Geological Survey. 
(b.) Woolhope Limestone.—In the original typical Upper Silurian 
tract of Shropshire, and the adjacent counties, the Upper Llandovery 
rocks are overlaid by a local group of grey shales containing nodular 
limestone which here and there swells out into beds having an aggregate 
thickness of 30 or 40 feet. These strata are well displayed in the 
picturesque valley of Woolhope in Herefordshire, which lies upon a worn 
quaquaversal dome of Upper Silurian strata rising in the midst of the 
surrounding Old Red Sandstone. They are seen likewise to the north- 
west at Presteign, Nash Scar, and Old Radnor in Radnorshire, and to 
the east and south in the Malvern Hills (where they include a great 
thickness of shale below the limestone), and May Hill in Gloucestershire. 
These strata have yielded many characteristically Upper Silurian fossils, 
including 13 genera and 24 species of crustacea and 17 genera and 56 
species of brachiopods. Among the common forms may be mentioned 
Bumastus Barriensis, Homalonotus delphinocephalus, Phacops caudatus, 
Atrypa reticularis, Orthis calligramma, Strophomena imbrex, Rhynchonella 
borealis, Rh. Wilsoni, Euomphalus sculptus, Orthoceras annulatum. 
It isa feature of the older Paleozoic limestones to occur in a very 
lenticular form, swelling in some places to a great thickness and rapidly 
