It is interesting and important to find that among the fossils of the oldest fossiliferous rocks the remains of molluscan shells occur, and that they are of kinds which can be satisfactorily referred to their place in the great series of the Mollusca. The most abundant of them are representatives of the Brachiopods or Lamp-shells. Among these are species of the genera Lingula (Lingulella, Fig.

FIG. 119.—Cambrian brachiopod (Lingulella Davisii, natural size).

119) and Discina which have a peculiar interest, inasmuch as they are the oldest known molluscs, and are still represented by living species in the ocean. They have persisted with but little change during the whole of geological time, from the early Paleozoic periods downwards, for the living shells do not appear to indicate any marked divergence from the earliest forms. They possess horny shells which are not hinged together by teeth. A more highly organised order of brachiopods possesses two hard calcareous shells articulated by teeth on the hinge-line. These forms, apparently later in their advent, soon vastly outnumbered the horny lingulids and discinids. So abundant are they both in individuals and in genera and species among the older Palæozoic rocks, that the period to which these rocks belong is sometimes spoken of as the "Age of Brachiopods."

The ordinary bivalve shells or Lamellibranchs had their representatives even in Cambrian times. From that early period they have gradually increased in numbers, till they have attained their maximum at the present time. Among the known Cambrian genera are Ctenodonta allied to the living "ark-shells," and Modiolopsis, probably representing some of the modern mussels.

The Gasteropods or common univalve shells, now so abundant in the ocean, made their advent not later than Cambrian time, for the remains of the genus Bellerophon (Fig. 129) are found in the group of strata known as the Lingula-flags in Wales.

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. Only scanty traces of cephalopodan life have yet been found among the Cambrian rocks. But occasional examples of the important genus Orthoceras (see Fig. 130) show that this great division of the molluscs had even in the earliest Palæozoic ages appeared upon the earth.

As the term Cambrian denotes, the rocks to which this name is applied are well developed in Wales. There, and in the border English counties, they attain a depth of perhaps more than 20,000 feet. They are found also in the east of Ireland, while in the north-west of Scotland they appear to be represented by massive red sandstones and conglomerates. The peculiar Primordial fauna has been widely recognised in both hemispheres. It occurs at intervals from France to Russia, and from Sweden to Bohemia and Sardinia. It is well represented in the United States and Canada, and it is met with even as far east as China.

The following Table gives the commonly accepted subdivisions of the Cambrian rocks in Britain.

Tremadoc group-dark grey slates.

Lingula Flags-bluish and black slates, flags, and sandstones.

Menevian group-sandstones, shales, slates, and grits. Harlech and Longmynd group-purple, red, and grey flags, sandstones, slates, and conglomerates.

CHAPTER XVIII

SILURIAN

THE origin and use of the term SILURIAN have already been given (p. 240). The rocks embraced under this term form a mass of strata which in some countries (Wales and Scotland) must be many thousand feet thick. Like the Cambrian system below, into which they graduate downward, they consist mainly of greywackes, sandstones, shales, or slates; but they are marked by the occasional occurrence of bands of limestone-a rock which from this part of the geological record appears in increasing quantity onwards to recent times. Some highly characteristic bands of dark carbonaceous shale are in some countries persistent for long distances, and contain abundant graptolites. Not infrequently these dark shales are full of pyritous impregnations, which, when the rock weathers, give rise to the efflorescence of alum or the formation of chalybeate springs; such bands are sometimes called alumschists. In Wales, the Lake District of the north of England, and to a less degree in the south of Scotland, there are remains of submarine volcanic eruptions of Silurian time in the form of intercalated sheets of tuff and beds of different lavas.

In certain regions (Russia, New York) Silurian rocks have undergone little change since the time of their deposition; but, as a rule, they have been more or less indurated, plicated, and dislocated (Wales, Lake District, etc.), while in some countries (Norway, Scotland) they have been so crushed and metamorphosed as to have assumed the character of schistose rocks (phyllites, mica-schists, etc.)

Murchison subdivided his Silurian system into two great sections, Lower and Upper. This classification still holds, though the limits and nomenclature of the several component groups have not been exactly maintained. The arrangement of the various

subdivisions, as followed in Britain, is shown in the table on p. 257.

Taking the fossils of the Silurian system as a whole, we find that they prolong and amplify the peculiar type of life found to characterise the Cambrian system. They include both flora and fauna. The flora, however, is exceedingly meagre. It consists almost entirely of sea-weeds, which occur usually in the form of fucoid-like impressions. But, as already remarked in reference to the so-called plants of the Cambrian rocks, many of the supposed vegetable remains are almost certainly not such (see p. 242). Some of them may be tracks left by worms, crustaceans, or other marine creeping or crawling creatures, upon soft mud or sand; others may be casts of hollows made by trickling water.or yielding sediment; while others seem to be the result of some peculiar crumpling or puckering of the strata. But undoubted remains of sea-weeds do occur. Some of these are delicate branching forms, like some still living, as shown in the organism figured in Fig. 120 from the Upper Silurian series. Among the Upper Silurian strata, also, traces of land-vegetation have been detected in the form of spores and stems of cryptogamous plants. Lycopods or club-mosses and ferns appear to have been the chief types in the earliest terrestrial floras; at least, it is remains referable to them that chiefly occur in the older Palæozoic rocks. They reached a great development in the Carboniferous period, in the account of which a fuller description of them will be given. We can dimly picture the Silurian land with its waving thickets of fern, above which lycopod trees raised their fluted and scarred stems, threw out their scaly moss-like branches, and shed their spiky cones.

The fauna of the Silurian period has been more abundantly preserved than that of the Cambrian, and appears to have been more varied and advanced. Among its simpler forms were Foraminifera and sponges. A foraminifer (of which there were no doubt representatives in Cambrian times, and there are still many living types in the present ocean, see Fig. 33) is generally a

minute animal, composed of a jelly-like substance which, possessing no definite organs, has in some kinds the power of secreting a hard calcareous or horny shell, through openings or pores (foramina) in which filaments from the jelly-like mass are protruded. By other kinds, grains of sand are cemented together to form a protecting shell. It is these calcareous and sandy coverings which occur in the fossil state and prove the presence of foraminifera in the older oceans of the globe. Sponges also are known to have existed in the Cambrian and Silurian seas, and their remains have been met with in all parts of the Geological Record down to the present day. It is, of course, only where these animals secrete hard durable parts that they can be detected as fossils. A sponge is a mass of soft, transparent, jelly-like substance, perforated by tubes or canals, and supported on an internal network of minute calcareous or siliceous spicules, or of interlacing horny fibres. Most fossil sponges are calcareous or siliceous, and their hard parts, being durable, have been preserved sometimes in prodigious numbers and in wonderful perfection. The common sponge of domestic use is an example of the horny type.

FIG. 121.-Graptolites from Silurian rocks. A, Rastrites Linnæi. B, Monograptus priodon. C, Diplograptus pristis. D, Phyllograptus typus. Murchisonii (all natural size).

E. Didymograptus

The Hydrozoa were abundantly represented in the Silurian seas by graptolites (see p. 242), of which there were many kinds. Some of the more characteristic of these are shown in Fig. 121. They abound in certain bands of shale, both in the Lower and