Mesozoic, or Secondary, the name applied to the great group of rooks between the Palaeozoic (q.v.) and the Cainozoic or Tertiary (q.v.), because the dominant types of living organisms preserved in them are intermediate between the types now existing and those so largely dissimilar which characterise the older sedimentary rocks. Among plants, cycads, conifers, and, towards its close, angiosperms, characterise this era; among animals, the Hexacoralla, abundant sea-urchins (Eu-echinoida), brachiopods abundant in species but of few genera, oysters, scallops, and numerous other bivalves, the cephalopodous Ammonites and Belemnites, homocercal fish (teleostean at the close of the period) and the enormous variety and size of reptiles, including crocodiles, turtles, and lizards, in addition to the great extinct groups, such as the Ichthyosauria, Plesiosauria, Pterosauria, and Dinosauria. The Mesozoic group comprises the three great systems, the Triassic, Jurassic, and Cretaceous, which are described separately.