| | | | Fishes, Invertebrates. | | +--------------+--------+------------------------------------+ | | |Primitive =Crossopterygii=, | | | Devonian. | Ganoidei, Dipnoi. Goniatites and | | | | other Invertebrates. Sharks. | |Palæozoic.+--------------+--------+------------------------------------+ | | | Upper. |Primitive =Sharks=. Ostracoderms. | | | | | Invertebrates. | | | Silurian. +--------+--------------------------------+---+ | | | Lower. |Blastids, Scorpions, Centipedes,| I | | | | | Sea-urchins and other | n | | | | | Invertebrates. | v | | +--------------+--------+--------------------------------+ e | | | |Marine =Worms=, Molluscs, | r | | | | Trilobites, Brachiopods, | t | | | Cambrian. | Crinoids, Star-fishes, | e | | | | Corals, Graptolites, | b | | | | Cystids, Sponges, | r | | | | Foraminifera. | a | +----------+-----------------------+--------------------------------+ t | | Archæan. |=Protozoans=, though there is | e | | | no evidence of life. | s | +----------------------------------+--------------------------------+---+ =Cambrian Period.= The Cambrian rocks have furnished many different species of marine animals. Seaweeds are the only plants found fossil in the Cambrian rocks. Some plants and animals may possibly have dwelt upon the land, but if so they have failed to leave any record of their existence. The animals are all _Invertebrates_, but not of extremely low forms; they have progressed since their appearance in an earlier period. Siliceous Sponges are not uncommon. The Cœlenterates are represented by Graptolites and Corals. Echinoderms are rare, and are principally represented by Cystids, a very primitive type; true Crinoids and Star-fishes appear before the close of the period. That marine worms existed is indicated by borings and tracks in the sands, which have since consolidated into rocks. Mollusca burrowed in the mud or crawled over the ocean bed. Brachiopods existed, and Arthropods were represented by primitive types. The most characteristic of Cambrian fossils are _Trilobites_ and _Lampshells_.