A geological period in the Palaeozoic era. It began about 359 million years ago, following the Devonian period, and extended until the beginning of the Permian period, about 299 million years ago. It is divided into the Mississippian and Pennsylvanian subperiods, dating before and after about 323 mya, respectively. During the Mississippian a marine transgression occurred and the characteristic rock of this division—the Carboniferous limestone—was laid down in the shallow seas. Fauna included foraminiferans, corals, ectoprocts, brachiopods, blastoids, and other invertebrates. The Pennsylvanian saw the deposition of the millstone grit, a mixture of shale and sandstone formed in deltaic conditions, followed by the coal measures, alternating beds of coal, sandstone, shale, and clay. The coal was formed from the vast swamp forests composed of seed ferns, lycopsids, and other plants. During the period fishes continued to diversify, amphibians became more common, insects diversified, and the first reptiles evolved.