Thus the Cambrian fauna, although it includes members of all the invertebrate phyla which occur as fossils, and although certain species are represented by multitudes of individuals, is none the less a restricted one, despite the high specialisation, probably of an adaptive nature, which certain of its members exhibit. There is a marked uniformity amongst the members of each of the great divisions, the range in structure presented by them being far smaller than that which is found at any later period of the earth's history. The occurrence of so many phyla, already sharply sepa rated from one another, and with their characteristic and typical structure fully developed, implies that the animal kingdom had had a long history in pre-Cambrian times, and the rarity of pre Cambrian fossils can only be explained by the fact that the early members of all phyla were unprovided with hard parts capable of ready preservation as fossils. That this explanation is justified, is indicated by the fact that, taken as a whole, the Cambrian animals themselves had delicate skeletons primarily of chitin, calcareous shells being rare and when found thin, when compared with the corresponding structures of Ordovician and later forms.
In the succeeding Ordovician and Silurian periods the range of structure amongst the members of each tribe becomes much greater, a fact which is reflected in the successive appearance during this time of new divisions, classes, orders and families, whilst the majority of those already existing in Cambrian times pass onwards, exhibiting increased specialisation as they do so. Thus amongst the Protozoa, Foraminifera make their appearance. Many groups of sponges appear and amongst the Coelenterates higher orders of graptolites are predominant over the Cambrian dendroids. The Anthozoa become represented by simple and then by compound corals belonging to the Tetracoralla, and the Tabulata represent the Aleyonaria. The Echinodermata display an enormous development of cystids, crinoids and asteroids and a few blastoids, edrioasteroids and echinoids appear. The Brachiopoda branch out in a similar manner, the articulate forms attaining a predominance both in numbers of species and in indi viduals. Amongst the Mollusca some 19 families of Lamelli branchia and a dozen of Gastropoda are first found, whilst the Nautiloidea are represented by about 2,000 species during the period. The Arthropoda undergo a similar expansion, new families of trilobites making their appearance, the Malacostraca becoming more abundant and varied, and the Merostomata exhibiting not only a great increase in individual size but also a marked adaptive radiation. Scorpions first appear in a form apparently not yet fully adapted for life on land. Finally perhaps at the top of the Ordovician, certainly in the Middle Silurian, the vertebrates ap pear in fish-like forms allied to the recent lampreys and hag fishes, and perhaps in true fishes belonging to the Acanthodi.
The succeeding Devonian period is rendered notable by the incoming of definite fishes. The Chondrichthyes are represented not only by acanthodians, but by forms more closely similar to the sharks, as well as by the heavily armoured Arthrodeira, whilst three distinct groups of bony fishes appear. Amongst the invertebrates, every phylum exhibits the addition of new families and higher groups, but the general character of the marine fauna remains the same, though the graptolites and trilo bites, the most characteristic of all Lower Palaeozoic animals, become rarer both as individuals and as species during Devonian times, one family after another of them becoming extinct. The Nautiloidea also become less abundant and less varied in their structure, their place being taken by members of the new order Ammonoidea, which make their first appearance in the Devonian or possibly towards the end of Silurian times. In the Devonian we get the first indication of true land-forms, the most striking being perhaps the centipedes from the Old Red Sandstone of Scotland.
In the Carboniferous we have a marine fauna which, whilst in the main representing a mere continuation of that in Devonian times, is none the less characterised by the dis appearance of the Receptaculida amongst the sponges, of many corals and of the graptolites amongst the Coelenterata, of some crinoids and all cistids, amongst the Echinodermata, and of many families of Brachiopoda. In their place we have a great expansion of the blastoids, asteroids and echinoids, and of both the lamelli branchs and gastropods among Mollusca, whilst the ammonoids become more abundant and more highly organised, passing up to forms which are the precursors of the great development which occurs in the Permian and Trias. Amongst the arthropods the trilobites survive only in a single family, but certain forms be longing to the Syncarida and Schizopoda represent a modern type of development of the Malacostraca The Merostomata are reduced in numbers and variety, but true scorpions and forms which are allied to the phalangers and spiders occur, together with representatives of the extinct group Anthracomarti. Myriapods become of gigantic size and insects are represented by a very large number of forms belonging to primitive orders, many of which bridge the gaps between existing groups. The heavily ar moured forms which represented the cyclostomes in Devonian times become extinct as do the arthrodeires. Elasmobranchs of many types are abundant and some of them exhibit the beginnings of structures which characterise the living Selachii. The bony fishes belonged to the three groups which appeared in Devonian times but one of them, the Palaeoniscidae has become far more abundant and varied in structure whilst the others are declining. The most important occurrence in Carboniferous times was the appearance of land-living and air-breathing vertebrates, Amphibia, and at any rate towards the end of the period, reptiles, Permian.—The marine fauna of the Permian is in the main a mere continuation of that which existed in Carboniferous times, but it is characterised by its poverty, perhaps only because marine deposits of Permian age are rare. Most of the characteristically Palaeozoic groups of invertebrates die out either during or at the end of Permian times. Amongst such groups are the Tetracoralla, all of the then existing families of crinoids, the blastoids and edrioasteroids and all the families of brachiopods, except those still extant, and four represented by rare forms in the Trias and the lower Jura. On the other hand all the families of Mollusca which were in existence in Carboniferous times pass on through the Permian into the Mesozoic and indeed in most cases still live. During this period the great order of the ammonoids undergoes an expansion, new families being introduced, whilst the members of the earlier groups become more elaborate in structure. The trilobites are represented by one genus only and that is restricted to the lower part of the system. The most characteristic features of the Permian fauna are displayed by the vertebrates. Amongst the fish, the elasmobranchs are represented by comparatively few forms, many of the more characteristic Carboniferous families having died out. The bony fish show a definite advance in the incoming of forms derived originally from a palaeoniscid stock which are the forerunners of the abundant Mesozoic ganoids. The Amphibia include the descendants of those which lived in the Carboniferous but were in many cases much more terrestrial in habits. The reptiles on the other hand become extraordinarily abundant and varied in structure, exhibiting many evolutionary series which lead on to the mammals and to the higher reptilian orders from which the birds arose. It is probable that in late Per mian times the reptiles were more varied than they have ever been since.