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IN TIME] As the highest of all vertebrates, it is natural to expect that mammals should be one of the latest groups of that assemblage to make their appearance; and this as a matter of fact they are, although it is by no means improbable that birds are the latest of all. Mammals are commonly stated to commence in the Trias, where they are presumed to be represented by Microlestes in Europe and by Dromatherium in North America. From the fact, however, that the approximately contemporary Tritylodon, which has cheek-teeth very like those of the former, appears to be in great degree intermediate between reptiles and mammals, it is by no means improbable that none of these Triassic creatures were true mammals. Undoubted mammals occur in the lower Jurassic Stonesfield Slate, in the upper Jurassic Permian beds, and, very sparingly, in the Wealden of England; while a large fauna has been discovered in the upper Cretaceous of North America. The mammals included among these Mesozoic forms appear, for the most part at any rate, to be referable to the Marsupialia, Insectivora, and, not improbably, the Monotremata (see ). After the lowest Eocene (when the Puerco fauna represented an inferior and apparently non-progressive type) mammals became abundant; and during that epoch most if not all of the existing orders made their appearance. The lower Eocene representatives of several of the orders, such as the Condylarthra among the Ungulata and the Creodonta among Carnivora, belonged, however, to low suborders which disappeared more or less completely by the Oligocene. Several subordinal groups of Ungulata developed and became extinct at later periods than the Eocene; but with the exception of the Ancylopoda and Tillodontia (whose right to ordinal distinction is by no means universally admitted), none of the Tertiary orders of mammals are extinct. At the present day—as during the greater part of the Tertiary epoch—mammals are the dominant terrestrial representatives of the Vertebrata. We have at present no evidence of the existence of Cretacea before the lower Eocene.

Although some of the three-toed bipedal tracks in the Trias of the Connecticut valley were formerly supposed to have been made by birds, there is little doubt that they are really due to dinosaurian reptiles. The class Aves, so far as we know, is therefore first represented by the long-tailed Archaeopteryx of the upper Jurassic, which represents a subclass (or order) by itself. Toothed birds also existed in the upper Cretaceous of both Europe and North America, but all these appear referable to existing ordinal (or subordinal) groups. By the lower Eocene, when teeth appear to have been entirely lost, most or all of the existing ordinal groups were developed, since which date the majority at all events have steadily increased.

In contradistinction to both the preceding classes, reptiles, which date from the Permian, are a waning group, at all events so far as both terrestrial and marine forms of large bodily size are concerned. The Permian reptiles were small or medium-sized creatures, few in number, and of generalized character. The one existing order dating from that epoch (when it was represented by Protorosaurus) is the Rhynchocephalia, of which the sole survivor is the New Zealand tuatara (Sphenodon). The Mesozoic period, from the Trias to the Chalk, is the true "age of reptiles," a number of orders being confined to that period. It is noteworthy, however, that the Triassic forms were in the case of the marine groups very generally of small size, and apparently amphibious, or perhaps freshwater. Of the various extinct Mesozoic orders, the Dinosauria, as demonstrated by their footprints in the sandstone of the Connecticut valley, were represented by species of huge size even in the Trias. The other extinct orders whose distribution was approximately coequal with the Mesozoic period were the ichthyosaurs (Ichthyopterygia), the plesiosaurs (Sauropterygia), and the pterodactyles (Ornithosauria). The Chelonia and Crocodilia (if we include the Phytosauria) date from the Trias, but are also dominant forms at the present day. But the mammal-like Anomodontia (Theromorpha), which ranged from South Africa to India and Russia, were solely Triassic. The Squamata, including lizards and snakes, together with the extinct Cretaceous Pythonomorpha (Mosasaurus, &c.), did not come into being till the upper Jurassic, or lower Cretaceous, and constitute the great bulk of the existing members of the class.

Batrachia, as represented by the labyrinthodonts, or Stegocephalia, carry the origin of vertebrates one tage further back, namely, to the upper Carboniferous. The stegocephalians, which appear to have included the ancestors of the anomodont reptiles, died out at the close of the Triassic epoch. The existing representatives of the class date, so far as is known, only from the Tertiary, but it is not improbable that the limbless caecilians (Apoda) may really be much older, since they appear to be related to the Stegocephalia.

The class Pisces is the lowest and at the same time the oldest representative of the Vertebrata, dating from the lower Ludlow beds of the upper Silurian. The oldest group is that of the sharks and rays (Elasmobranchii), in which the orders Pleuropterygii, Ichthyotomi and Acanthodii are confined to the Palaeozoic. The lung-fishes (Dipnoi) are also an ancient group, although surviving in the form of three genera widely

sundered in space; the order Arthrodira (as represented by Coccosteus of the Devonian) was solely Palaeozoic. Of the subclass Teleostomi, the fringe-finned group (Crossopterygii) attained its maximum in the Palaeozoic, although it survives in the shape of two African genera. In the case of the subclass Teleostomi the enamel-scaled, or ganoid, division was abundant during the Palaeozoic and early Mesozoic periods (and still survives in North America), but the modern soft-scaled bony fishes did not make their appearance till the Cretaceous, or thereabouts.

Of the class Agnatha, as typified by the modern lampreys, the palaeontological record is very imperfect. There is, however, an armoured subclass, the Ostracodermi, represented by Pterichthys, Cephalaspis, &c., which was confined to the upper Silurian and Devonian; and Palaeospondylus of the Devonian has been regarded as an early lamprey (Cyclostomi). Whether the so-called conodonts, ranging from the upper Cambrian to the Carboniferous, are really teeth of lampreys, has not been definitely ascertained.

The lamp-shells, or Brachiopoda, form an exceedingly ancient group, dating from the lower Cambrian, and surviving at the present day, although in greatly diminished numbers compared to the Palaeozoic epoch, when they far surpassed the now dominant bivalve molluscs. The group attained its maximum in the Silurian, when, as in the Palaeozoic genera, nearly all the forms belonged to the hingeless section. With the beginning of the Mesozoic period the waning of the brachiopods, which had set in with the Devonian, became more pronounced, and was continued throughout the Mesozoic formations. A remarkable feature is the survival to the present day of the Cambrian genera Lingula, Discma and Crania (or closely allied types).

The Polyzoa, of which the sea-mats (Flustra) are well-known representatives, date from the Ordovician; the Palaeozoic forms belonging almost exclusively to the section Cyclostomata, which attained its maximum in the Jurassic; while the dominant modern Chilostomata came in with the Tertiary.

The Mollusca, of which the great bulk are marine and the majority of the remainder freshwater, are perhaps the most important of all fossils from the chronological point of view. Since the three principal classes (Pelecypoda, Gastropoda and Cephalopoda) are represented in the upper Cambrian, it is evident that the origin of the group was much earlier. In the Palaeozoic the chambered cephalopods of the section Tetrabranchiata (now represented by the nautilus) were the dominant forms; the bivalves (Pelecypoda) and gastropods showing a relatively poor development. The tetrabranchiate cephalopods continued throughout the Mesozoic, when they were specially represented by the ammonites; but by the Tertiary they had become almost extinct. The section Dibranchiata (cuttle-fishes) commenced with the Mesozoic. The bivalves and gastropods have shown a steady increase to the present day, and are now the dominant forms.

Insects date from the Ordovician graptolite-slates of Sweden, where they are represented by Protocimex; the next oldest being Palaeoblattina of the French upper Silurian. From the Devonian about a dozen forms are known, belonging to several groups; and from the Coal-measures extensive insect-faunas have been described. All the Palaeozoic forms lack most of the distinctive features by which the modern groups are characterized, the majority of them showing kinship to the cockroach group.

The Myriapoda (centipedes and millipedes) are of comparatively little importance as fossils. The class dates from the Devonian, and is abundant in the Coal-measures; the Palaeozoic forms for the most representing two orders—Archipolypoda and Protosyngnatha—peculiar to that period, of which the second has only a single known species. The modern centipedes (Chilopoda) date mainly from the Tertiary, although several Carboniferous genera have been assigned to the group. Millipedes (Diplopoda), although known from the Cretaceous of Greenland, elsewhere date from the Tertiary.

The class Arachnida, now taken to include trilobites and king-crabs, as well as scorpions and spiders, is ancient. Scorpions—not far removed from existing types—are known in the Silurian, while true spiders occur in the Coal-measures. The great majority of the more typical Palaeozoic arachnids are, however, referred to an order by themselves—the Anthracomarti. King-crabs (Xiphosura) date from the Silurian, the existing genus Limulus occurring in the Trias; but the gigantic eurypterids (Eurypterida) and the trilobites (Trilobita) are exclusively Palaeozoic, the former dating from the Ordovician, and the latter from the upper Cambrian.

Most of the existing ordinal groups of the class Crustacea appear to date from the Palaeozoic; the decapods (lobsters and crabs) which represent the highest development of the class, did not, however, attain a dominant position till well on in the Mesozoic, and are at their maximum in the present day; genuine crabs (Brachyura) apparently not having come into existence till the Cretaceous.

Among the Echinodermata, the classes Blastoidea and Cystoidea are exclusively Palaeozoic, while the stone-lilies (Crinoidea) form a