Lingula-flags in Wales. They abounded in the Silurian sea, upwards of 1300 species having been found in Silurian rocks. Among the more frequent genera are Bellerophon, Ophileta, Holopæa, Murchisonia, Platyschisma.

The highest division of the molluscs, the Cephalopods, to which the living nautilus and cuttle-fish belong, is but poorly represented at the present time. But during the Palæozoic and Secondary periods it flourished exuberantly, both as regards number of individuals and variety of forms. It is divisible into two great families. In one of these the shell is usually internal and is never chambered; in the other the shell is chambered and external, the chambers being connected by a tube or siphuncle. The former family includes all the living cuttle-fishes, squids, and the paper nautilus; the latter comprises only one living representative -the pearly nautilus. It is to the family of chambered cephalopods that the Palæozoic forms are all referable. In some the shell was straight, in others it was variously curved. Among the Silurian genera were Orthoceras (Fig. 130, straight); Cyrtoceras (curved); Ascoceras (globular or pearshaped); Lituites (Fig. 130, coiled), and also Nautilus, a genus which has persisted through the greater part of geological time down to the present day, and now remains the only representative of the chambered cephalopods formerly so abundant.

Remains of Fishes detected in the Upper Silurian rocks are the earliest traces of vertebrate life yet known. They consist partly of plates which are regarded as portions of the bony covering of certain placoderms or bone-plated forms (Pteraspis, Cephalaspis, Auchenaspis); partly of curved spines and shagreen-like fragments. These creatures appeared as forerunners of the remarkable assemblage of fishes that characterised the next geological period (see p. 340). All the animal remains here enumerated are relics of the in

FIG. 130. Silurian Cephalopods; (a) Orthoceras emeritum(); (b) Trochoceras (Lituites) cornu-arietis (3).

habitants of the sea. Of the land-animals of the time nothing was known until the year 1884, when, by a curious coincidence, the discovery was made of the remains of scorpions in the Silurian rocks of Sweden, Scotland, and the United States, and of an insect allied to the living cockroach (Palæoblattina) in those of France. If scorpions and insects existed during this ancient period we may be sure that other forms of terrestrial life were also present. A new interest is thus given to the prosecution of the search for fossils among these older formations.

Putting together the evidence furnished by the rocks and fossils of the Silurian system, we get a glimpse of the aspect of the globe during the early geological period which they represent. The rocks bring before us the sand, mud, and gravel of the bottom of the sea, and tell of some old land from which these materials were worn away. The detritus carried out from the shores of that land was laid down upon the sea-bottom just as similar materials are being disposed of at the present day. The area occupied by Silurian rocks marks out the tracts then covered by the sea. Following these upon a map we perceive that vast regions of the existing continents were then parts of the ocean-floor. In Europe, for example, Silurian rocks underlie the greater part of the British Islands, whence they stretch northwards across a large part of Scandinavia and the basin of the Baltic. They rise to the surface in many places on the continent from Spain to the Ural Mountains. They are found forming parts of some of the great mountain-chains of the globe, as, for instance, in the Cordilleras of South America, in the Alps, and in the Himalayas. Even at the antipodes they are met with as thick masses in Australia and New Zealand. It is evident that the geography of the globe in Silurian times was utterly unlike what it is now. large part of the present land was then covered with shallow seas, in which the Silurian sedimentary rocks were laid down. There would seem to have been extensive masses of land in the boreal part of the northern hemisphere connecting the European, Asiatic, and American continents. Along the coast-line of the northern land and across the shallow seas lying to the south of it, the same species of marine organisms migrated freely between the Old and the New Worlds.

A

The following Table shows the subdivisions which have been made in the Silurian system of Britain.

Upper
Silurian.

Ludlow group (mudstone and Aymestry limestone)-Kirkby Moor and Bannisdale flags and slates.

Wenlock group (shales and limestones)-Denbighshire and Coniston grits and flags.

Upper Llandovery group-May Hill sandstones.

Lower Silurian.

Lower Llandovery group-grits and sandstones.

Bala and Caradoc group-sandstones, slates, and grits, with

Bala (Coniston) limestone.

Llandeilo group-dark argillaceous and sometimes calcareous

CHAPTER XVIII.

DEVONIAN AND OLD RED SANDSTONE.

THE Devonian system, which comes next in order, was named by Sedgwick and Murchison after the county of Devon where they studied its details. In Europe, and likewise in the eastern part of North America, it occurs in two distinct types which bring before us the records of two very different conditions in the geography of these regions during the time when the rocks composing the system were being deposited. The ordinary type, which occurs all over the world, represents the tracts that were covered by the sea, and has preserved the remains of many forms of the marine life of the period. It is that to which the name Devonian is more particularly applicable. The less frequent type is characterised by thick accumulations of sandstones, flagstones, and conglomerates laid down in lakes and inland seas, and contains a very distinct assemblage of land and fresh-water fossils. This lacustrine type is known by the name of Old Red Sandstone.

In their general character the Devonian rocks resemble those of the Silurian system underneath. In Central Europe, where they attain a thickness of many thousand feet, their lower division consists mainly of sandstones, grits, greywackes,

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