of the whole of the Lower Palæozoic formations, present considerable uniformity over the whole globe. They consist of grey and reddish grits, sandstones, greywackes, quartzites, and conglomerates, with thick groups of shale, slate, or phyllite. These sedimentary accumulations attain a great thickness in some countries. In Wales they have been estimated by some observers to be at least 20,000 feet in depth. Their ripple-marks, pebble-beds, and frequent alternations of coarse and fine sediment, point to their having probably been laid down in comparatively shallow water, during a period of prolonged subsidence of the sea-bottom. They include tuffs and basic lavas which indicate contemporaneous submarine eruptions.

With regard to the occurrence of fossils among the older Palæozoic formations, and indeed among stratified rocks in general, it is worthy of notice that they are far from being equally distributed; that, on the contrary, they occur by preference in certain kinds of material rather than in others. Grits and sandstones, for instance, are comparatively unfossiliferous, while fine shales, slates, and limestones are often crowded with fossils. It is not that life was probably on the whole more abundant at the time of the deposition of some kinds of strata, but that the local conditions for its growth and for the subsequent entombment and preservation of its remains were then more favourable. At the present time, for example, dredging operations show the most remarkable variations between different and even adjacent parts of the sea-bottom as regards the abundance of marine life. Some tracts are almost lifeless, while others are crowded with a varied and prolific fauna. We can easily understand that if, from the nature of the bottom, plants and smaller animals cannot flourish on a particular tract, the larger kinds that feed on them will also desert it. Even if organisms live and die in some numbers over a part of the sea-bed, the conditions may not be suitable there for the preservation of their remains. The rate of deposit of sediment, for instance, may be so slow that the remains may decay before there is time for them to be covered up; or the sediment may be unfitted for effectually preserving them, even when they are buried in it. We must not lose sight of these facts in our explorations of the Geological Record. A relation has always existed between the abundance or absence of fossils in a sedimentary rock and the circumstances under which the rock was originally formed.

The oldest fossiliferous strata (Cambrian or Primordial) contain a remarkable assemblage of animal remains, which, being the

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earliest traces of the animal life of the globe, might have been anticipated to belong to the very lowest tribes of the animal kingdom. But they are by no means of such humble organisation. On the contrary, they include no representatives of many of the groups of simpler invertebrates, which we may be sure were nevertheless living at the same time. Not only so, but some of the fossils belong to comparatively high grades in the scale of invertebrate life, such as chambered molluscs. From this incompleteness, and from the wide differences in the organic grade of the forms actually preserved in the rocks, we may reasonably infer that only a most meagre representation of the life of the time has come down to us in the fossil state. Some of the fossils, moreover, have been so indistinctly preserved that considerable difficulty is experienced in deciding to what sections of the animal or vegetable kingdoms they should be assigned.

Among the markings which have given rise to much discussion allusion may be FIG. 115. Fucoid-like impres- made to plant-like impressions, some of sion(Eophyton Linneanum) which, like Eophyton (Fig. 115), have been from Cambrian rocks (1).1 claimed as sea-weeds. Others, however, may only be irregular wrinklings of the surfaces of deposit, and of no organic origin at all (see p. 249). Another puzzling impression is that called Oldhamia (Fig. 116), which has been variously referred to the Hydrozoa, the Sertularia, the Polyzoa, and the calcareous Algae.

Some of the most characteristic older Palæozoic organisms belong to the Hydrozoa, and are embraced under the general title of Graptolites-a name given to them from their fancied resemblance to quill-pens. They were composed of a horny or chitinous substance, and hence they commonly present themselves merely as black streaks upon the stone. Each graptolite was a colony comprising many individuals which occupied each its own cell. The cells are in some kinds placed in a row on one side of a supporting rod or axis; in other kinds there is a row of cells on both sides (see Fig. 121). Some varieties are straight, others curved

FIG. 116. Oldhamia radiata (natural size), Ireland.

or spiral. Some are simple branches, others are composed of two or more branches, while in certain types a large number of separate branches is united in one common centre. One of the most

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ancient hydrozoa is Dictyograptus (Dictyonema, Fig. 117) —а characteristic fossil of the Cambrian rocks of Scandinavia. The graptolites are more especially characteristic of the Silurian system (p. 250).

The Echinodermata had their representatives in Cambrian time, though the remains of these are few and infrequent. The great tribe of the Crinoids or Sea-lilies (p. 251) had already established itself on the floor of the Cambrian sea, where also there were representatives of Cystideans and star-fishes (see p. 252).

Numerous kinds of Sea-worms (Annelids) crawled over the sandy and muddy bottom and shores of the Cambrian ocean. These creatures have left no trace of their bodies, which, like those of their representatives in the present ocean, were soft and unfitted for preservation. But the burrows they made in wet sand or mud, and the trails they left upon the soft surfaces over which they moved, have been abundantly preserved (see Fig. 124). These

FIG. 118. Cambrian Trilobites. (a) Paradoxides bohemicus (natural size); (b) Agnostus princeps (f); (c) Olenus micrurus (natural size); (d) Ellipsocephalus Hoffi (natural size).

markings afford unquestionable proof of the presence of creatures which have otherwise utterly disappeared.

Among the most abundant and characteristic fossils of the older stratified rocks of the earth's crust are those to which the general name of Trilobites has been given. These long-extinct animals were crustaceans, having a more or less distinctly threelobed body, at one end of which was the head or cephalic shield, usually with a pair of fixed compound eyes; at the other end the caudal shield or tail; while between the two shields was the ringed or jointed body, the rings of which were movable, so that the animal could bring the two shields together or coil itself up. It will be seen from the different genera represented in Figs. 118 and 125 how varied were the forms which they assumed. In the shapes and relative sizes of the shield and segmented body, in the number of the body-rings, in the development of spines, and in other features, the most wonderful variety is traceable among the trilobites even of the oldest fossiliferous strata. Some of the earliest genera were also the largest; Paradoxides sometimes reaching a length of nearly two feet. Yet contemporaneous with this large creature were some diminutive forms. A few genera (among them Agnostus) were blind; but most possessed eyes furnished with facets, which in some forms are fourteen in number, while in others they are said to amount to 15,000. The peculiar crescent-shaped eye on each side of the head is well shown in some of the forms represented in Figs. 118 and 125. The trilobites appear to have particularly swarmed on sandy and muddy bottoms, for their remains are abundant in many sandstones and shales.

Another form of crustacean life represented in the early Palæozoic ocean was that of the Phyllopods-animals furnished with bivalve shell-like carapaces, which protected the head and upper part of the body, while the jointed tail projected beyond it. Most of them were of small size (see Fig. 126). The characteristic Cambrian genus is Hymenocaris.

Of all the divisions of the animal kingdom none is so important to the geologist as that of the Mollusca. When one walks along the shores of the sea at the present time, by far the most abundant remains of the marine organisms to be there observed are shells. They occur in all stages of freshness and decay, and we may trace even their comminuted fragments forming much of the white sand of the beach. So in the geological formations, which represent the shores and shallow sea-bottoms of former periods, it is mainly remains of the marine shells that have been preserved. From their abundance and wide diffusion, they supply us with a basis for the comparison of the strata of different ages and countries, such as no other kind of organic remains can afford.