CLASSIFICATION OF THE SILURIAN ROCKS. YT 
Strophomena arenacea, St. imbrex, Leptena Grayi, Chonetes lata, Atrypa hemispherica, 
Spirifer elevatus, &c. The Lamellibranchiata and Gasteropoda contain genera and species 
hitherto unknown; added to which, /Wenopsis Thomsoni, Illenus Barriensis, Hucrinurus 
punctatus, Proetus Stokesii, and Phacops Downingia, among the Trilobites, offer other 
features of distinction, and tend still more to elucidate the clear succession in time of these 
Llandovery Beds; such forms being here found for the first time im the ascending series 
of deposits. 
When we mount into the true Upper Silurian Series, as constituting the 
Wenlock and Ludlow Formations, we find ourselves generally in rocks of the same 
argillaceous character as in the Lower Silurian, but usually im a much less hardened 
and altered state; so much so, that in many tracts where limestones do not prevail, 
the beds are little more than slightly indurated, finely levigated mudstones, which 
have not undergone the action of metamorphism that has affected them in other. 
localities with a slaty cleavage, and thus altered their aspect. But, in Great Britain, 
the phenomena of unaltered strata is very much confined to the typical Silurian region ; 
for in parts of North Wales some of the Wenlock Shales have been converted into roofing 
slates. This also holds good in many other parts of the world. Indeed, in Devonshire 
and Cornwall, and in the Rhenish provinces, strata of the Devonian age are equally slaty. 
As might be expected, the great abundance of the Upper Silurian fossils occurs in the 
zones of subordinate limestone ; and hence the Woolhope or Lower Wenlock Limestone, 
and the Wenlock or Dudley Limestone, are the centres, particularly the last mentioned, 
wherein the reliquize of the marine animals of that period most abound. In these beds 
the great mass of the Silurian Brachiopoda figured and described by Mr. Davidson occur, 
associated with many genera of Crustacea common to the Lower Silurian, some being 
entirely new forms ; Corals and Echinodermata, as well as the Cephalopoda, having also 
much increased. Amongst the Corals (Coelenterata) many new genera now appear, 
comprising Acervularia, Aulacophyllum, Conites, Omphyma, Cystiphyllum, &c. The 
Echinodermata seemed to have reached their maximum development in the seas of this 
age; and, although too numerous to mention here, I must allude to the Cystidez, 
Caryocystites, Prunocystites, Pseudocrinites, &c., a remarkable group,  stratigraphi- 
cally and zoologically defined by the late Leopold von Buch and the late Professor 
Edward Forbes. These Crinoids are associated in the same beds with the lily-shaped 
Encrinites, Crotalocrinus, Cyathocrinus, Dimerocrinus, Ichthyocrinus, and Taxocrinus, &c. ; 
also with those Cephalopoda characterised by numerous species of the group of the 
Orthoceratide, many of which assumed gigantic proportions. Yet, amidst this vast 
profusion and wealth of marine life no trace of a true Fish, I repeat, has hitherto been 
found in strata of this or the preceding age in any country. 
‘Towards the close of these Silurian strata, as seen near Ludlow, the argillaceous 
and calcareous beds pass upwards into more sandy and shore-like deposits, and with 
this lithological change the earliest known Fishes appear (see JVo¢e, p. 30), and thus 
