Foraminifera are known to have existed in the Ordovician, and were abundant in many Pennsylvanian limestones. Num mulites abounded in the Eocene limestones. Orbitoides, which formed the Vicksburg limestone, are known from the Cretaceous to the Miocene.
The subclass Actinopoda contains animals that depend on floating for locomotion. Radiolaria, the most important mem bers of this class, arc marine organisms, and when accumulating on sea bottom, form silicious deposits known as "radiolarian ooze" and became important rock builders in the Tertiary; traces of them are found in nearly all periods.
The other classes which are Mastigophora, Sporozoa and Infusoria are unimportant as fossils.
The typical animals of the Porifera are the sponges, are aquatic animals generally fixed to some object and found mostly in the shallow parts of the seas; only those having a pilicious skeleton occur at great depth. Those having a calcareous skeleton are known as Calcispongiae, while those with a silicious test are Silicispongiae.
These are mostly marine animals with a body cavity which is the digestive organ. The mouth is surrounded by numerous tentacles. In the Hydrozoa subdivision are the polyps and medusae, with an important order of Graptolites, which are well developed in _the upper Cambrian, Ordovician and Silurian; only a few are known in the Mississippian. The class of Scy phozoa, in which are the jelly-fish composed mostly of water and have no hard parts, therefore, poorly preserved as fossils. The third class are the Anthozoa, or corals, which are found mostly in shallow seas, and at present may be found along the Atlantic, but not in the colder waters of the North.
In these phyla the fossils are poorly preserved and are unim portant in a geological sense.
Echinodermata are marine animals having a calcareous skeleton or plates generally arranged in fives (pentamerous), and may be distinguished from the Coclenterata as they have a digestive tube separate from the body cavity, and more highly developed nervous system. The various important classes are (A) Cystoidea or Cystoids, (B) Blastoidea or Sea-Buds, abun dant in the Devonian and entirely extinct at the end of the Car boniferous; (C) Crinoidea or Sea-Lilies; (D) Asteroidea or Star-Fish; (E) Ophiuroidea or Brittle-Stars and Serpent Stars; (F) Echinoidea or Sea-Urchins; (G) Holothuridoidea or Sea Cucumbers.
• These are usually marine animals having their soft parts and well-developed digestive canal enclosed in a calcareous, horny or membranous covering.
The hard outer covering of the Bryozoa are the only parts found as fossils. Phoronida are wormlike marine animals unknown in the fossil state. Brachiopods are found living in shallow water, having calcareous shells or valves with concentric lines of growth. Brachiopods may be distinguished from the Pelecypods (of the next phylum) as they are equilateral, while Pelecypods are incquilateral; in Brachiopods the two valves are never alike while in Pelecypods they are nearly or exactly alike. Brachiopods are about i• to ri inches in size, although a few are much larger, and Productus giganteas of the Mississippian sometimes reach one foot in size. Inarticulata of the Lingula subgroup are evidence of shallow water conditions as the shallow water forms are the most prolific in the fossil state.
,Mollusks are covered, shell animals able to swim, crawl and burrow; the various classes that are distinguished are the (A) Amphineura, which are marine animals with a wide range of depth. (B) Pelecypoda or clam group, sometimes classified as Lamellibranchs. (C) Gastropoda, the familiar snails are examples of this class. They have a rather large calcareous shell, with six whirls or coils at the posterior end and prolonged into a half cylindrical canal at the anterior end. The gastropods have a distinct head as well as tentacles and eyes. (D) Scaphopoda have a calcareous shell, curved and tapering, the larger end being anterior, which has the head and feet but no eyes. They are usually found at great depth in the seas. (E) Cephalopoda. One order of this branch, the Tetrabranchiata (four gills) have an external shell of many chambers but only the last one being inhabited; the second order is the Dibranchiata (two gills) have an internal shell or it may be entirely wanting. The head bearing eight or ten arms encircling the mouth. Cephalopods were the strongest competitors of the Vertebrates, and were known in the Cambrian with maximum development in the Silurian, declining towards the Triassic. The Ammonites of the order Tetrabranchiata are usually coiled in closely, in a flat spiral and are important "index-fossils" of the Mesozoic. The Belemnites. of the order Dibranchiata are also important as index of the Jurassic and Cretaceous.