Cyclocrinitids are marine dasycladaceous algae of Ordovician and Silurian age. Their skeletal elements consist of a seldom calcified main axis on which lateral branches are borne in whorls. Calcified heads commonly form at the termini of the laterals. In North America cyclocrinitids are represented by three genera: Anomaloides, Cyclocrinites, and Lepidolites. Anomaloides (=Anomalospongia) possesses laterals calcified throughout their extent. They expand gently outward, and no heads form. Three secondary laterals form threadlike projections at the terminus of each lateral. -- Cyclocrinites (=Cyclocrinus, Lunulites, Cerionites, Pasceolus, Mastopora, and Nidulites) with a globose thallus is the most diversified genus. The laterals expand at the termini and form generally six-sided heads. In one species the laterals branch to the second degree. The lateral heads are in some species supported by four to six ribs. In at least one species the laterals constrict twice and form two layers of heads, one above the other. The main axis is generally short, and attachment is by means of a pedicle that is, however, frequently not preserved. Calcification generally occurs above and below the lateral heads. One new species, Cyclocrinites welleri, is described. -- Lepidolites consists of one species only and is the most modified of all cyclocrinitids. The laterals are short, small, and calcified; their ends are modified and form overlapping plates. -- The cyclocrinitids have been variously assigned by many authors to many groups, particularly as an appendix to the sponges. They are here considered a basal receptaculitid stock that possesses the simplest structures. They fill an important gap in the fossil record of Paleozoic algae Includes bibliographical references (p. 165-182) Pt. I: Introduction -- Previous major publications -- Morphology -- Taxonomic position -- Ecology -- Pt. II: Systematic descriptions -- Pt. III: Original definitions and descriptions of other authors. -- Annotated bibliography Cyclocrinitids are marine dasycladaceous algae of Ordovician and Silurian age. Their skeletal elements consist of a seldom calcified main axis on which lateral branches are borne in whorls. Calcified heads commonly form at the termini of the laterals. In North America cyclocrinitids are represented by three genera: Anomaloides, Cyclocrinites, and Lepidolites. Anomaloides (=Anomalospongia) possesses laterals calcified throughout their extent. They expand gently outward, and no heads form. Three secondary laterals form threadlike projections at the terminus of each lateral. -- Cyclocrinites (=Cyclocrinus, Lunulites, Cerionites, Pasceolus, Mastopora, and Nidulites) with a globose thallus is the most diversified genus. The laterals expand at the termini and form generally six-sided heads. In one species the laterals branch to the second degree. The lateral heads are in some species supported by four to six ribs. In at least one species the laterals constrict twice and form two layers of heads, one above the other. The main axis is generally short, and attachment is by means of a pedicle that is, however, frequently not preserved. Calcification generally occurs above and below the lateral heads. One new species, Cyclocrinites welleri, is described. -- Lepidolites consists of one species only and is the most modified of all cyclocrinitids. The laterals are short, small, and calcified; their ends are modified and form overlapping plates. -- The cyclocrinitids have been variously assigned by many authors to many groups, particularly as an appendix to the sponges. They are here considered a basal receptaculitid stock that possesses the simplest structures. They fill an important gap in the fossil record of Paleozoic algae Fieldiana series has been published as Geological Series by Field Columbian Museum (1895-1909) and Field Museum of Natural History (1909-1943), and as Fieldiana: Geology by Chicago Natural History Museum (1945-1966) and Field Museum of Natural History (1966-)