the most wonderful variety is traceable among the trilobites even of the oldest fossiliferous strata. Some of the earliest

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b

a

d

FIG. 124. Trilobites (Primordial or Cambrian); (a) Paradoxides Bohemicus (natural size); (b) Agnostus princeps(); (c) Olenus micrurus (natural size) ; (d) Ellipsocephalus Hoffi (natural size).

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. 124 and 125. The trilobites appear to

have particularly swarmed on sandy and muddy bottoms, for their remains are abundant in many sandstones and shales.

FIG. 125. Trilobites (Lower and Upper Silurian); (a) Asaphus tyrannus (3); (b) Ogygia Buchii(); (c) Illanus barriensis (3); (d) Trinucleus concentricus (natural size); (e) Homalonotus delphinocephalus ().

They continued to flourish all through the Silurian period.

But towards the close of Palæozoic time they died out. They are thus a distinctively Palæozoic type of life, each great division of the Palæozoic rocks being characterised by its own varieties of the type.

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 (Fig. 126).

Of all the divisions of the animal kingdom none is so imporFIG. 126. Silurian Phyllo- tant to the geologist as that of the pod Crustacean (Ceratio- Mollusca. When one walks along

caris papilio).

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.

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. 127) and Discina which

FIG. 127. Silurian Brachiopods; (a) Lingulella Davisii (natural size), Cambrian; (b) Atrypa reticularis (natural size), Caradoc beds to Lower Devonian; (c) Orthis actonia (natural size); (d) Rhynchonella borealis (natural size); (e) Pentamerus galeatus (natural size).

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 Palæozoic 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 hingeline. 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." Common Silurian genera are Orthis, Atrypa, Pentamerus, Leptana, and Rhynchonella (Fig. 127).

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

FIG. 128.- Silurian Lamelli- FIG. 129.-Silurian Gasteropod branch (Orthonota semisulcata)

(natural size).

have attained their maximum at the present time. Among the Silurian genera are Ctenodonta allied to the living "arkshells," Orthonota (Fig. 128), 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