The Mesozoic era, so far as we can judge, seems to have been shorter than the Palaeozoic; in North America Mesozoic rocks are very much more important and widely spread in the western half of the continent than in the eastern. The latter region was, in a measure, completed by the Appalachian revolution, and subsequent growth consisted merely in the successive addition of narrow strips to the coast-line, but in the West many great changes were required to bring the land to its present condition.

The life of the Mesozoic constitutes a very distinctly marked assemblage of types, differing both from their predecessors of the Palaeozoic and their successors of the Cenozoic. In the course of the era the Plants and marine Invertebrates attained substantially their modern condition, though the Vertebrates remain throughout the era very different from later ones. Even in the Vertebrates, however, the beginnings of the newer order of things may be traced. In the earlier two periods, the Triassic and Jurassic, vegetation is almost confined to the groups of Ferns, Cycads, and Conifers, but with the Cretaceous come in the Angiosperms, both Monocotyledons and Dicotyledons, and since then the changes have been merely in matters of detail.

With few exceptions, the ancient Tetracoralla had all disappeared, and the modern Hexacoralla took their place. The Echinoderms were all markedly different from the Palaeozoic types. The Cystoids and Blastoids had died out, and the Crinoids had been revolutionized, the Camerata being replaced by the Articulate Likewise the modern sea-urchins, Euechinoidea, replaced the ancient Palceechinoidea, and many Mesozoic genera of the former group are still living in our modern seas. The Starfishes also assumed their modern condition. Brachiopods were far less abundant and diversified than they had been in the Palaeozoic, and belonged, for the most part, to different families, while the Bivalve and Gastropod Mollusca increased to a wonderful extent. Especially characteristic are the marvellous wealth and variety of the Am-monoid Cephalopods, which disappear at the close of the era. The Dibranchiate Cephalopods, with internal shells, make their first appearance in the Mesozoic, and one group of them, the Belemnites, is almost exclusively confined to the era. The Arthropods showed the same revolutionary changes.

Among the Crustacea, the Trilobites and Eurypterids have gone out, but all the modern groups were well represented, though many of the Mesozoic genera are no longer to be found in the seas of to-day. Insects reached nearly their modern condition, so far as the large groups are concerned, butterflies, bees, wasps, ants, flies, beetles, etc., being added to the older orthopters and neuropters.

Fishes became modernized before the close of the era, the Bony Fishes having acquired their present predominance. The Amphibia took a subordinate place, and after flourishing for a time, the great Stegocephalia died out, leaving only the pygmy salamanders and frogs of the present. Birds and Mammals made their first appearance, the former advancing rapidly to nearly their present grade of organization, though not reaching their present diversity, while the mammals remained throughout the era very small, primitive, and inconspicuous. The most significant and characteristic feature of Mesozoic life is the dominance of the Reptiles, which, in size, in numbers, and in diversified adaptation to various conditions of life, attained an extraordinary height of development. The Mesozoic is called the "Era of Reptiles," because these were the dominant forms of life. They filled all the roles now taken by birds and mammals; they covered the land with gigantic herbivorous and carnivorous forms, they swarmed in the sea, and, as literal flying dragons, they dominated the air. At the present time there are only five orders of reptiles in existence, and of these only the crocodiles and a few snakes attain really large size.

In the Mesozoic era no less than twenty-five reptilian orders flourished, and many of them had gigantic members. Some were the largest land animals that ever existed, and the sea-dragons rivalled the whales in size. Nothing so clearly shows that the Mesozoic era is a great historical fact, as the dominance of its reptiles.

The Mesozoic climates offer some difficult problems. In general, the climate was mild, as is shown by the plants found in the Mesozoic rocks of Arctic lands, for in Greenland, Alaska, and Spitz-bergen was a luxuriant vegetation of warm temperate type. On the other hand, certain geologists have maintained the existence of distinct climatic belts in the Mesozoic, indicating equatorial, northern, and southern zones, but by others this interpretation is denied.

The Mesozoic era comprises three periods, - the Triassic, Jurassic, and Cretaceous.