Five new edrioasteroid genera from the Early and Middle Ordovician of western Utah greatly enlarge the record of this scarce echinoderm class during its early diversification. Data from a diversity of taxa were used to generate a new phylogeny and classification of the edrioasteroids. New and reinterpreted morphology, particularly for the aboral surface, is introduced. Forty-two characters were scored for 15 taxa and subjected to a PAUP 3.0 parsimony analysis. This analysis identified two major edrioasteroid clades with subsequent subbranchings. One major clade is the isorophids, whose ancestry can be traced to the Early Cambrian; the second major clade includes edrioasterids and the former class Edrioblastoidea, dating to the Middle Cambrian. Edrioblastoids, rhenopyrgids, and cyathocystids mapped as specialized branches of the edrioasterid clade, while pyrgocystinids mapped as highly derived lebetodiscid isorophids. The cladistic analysis supported our hypothesis for analogous thecal elongation structures among edrioasterids and isorophids. -- The fauna contains the edrioasterid Paredriophus elongatus, n. gen. and sp., the edrioblastoid Lampteroblastus hintzei, n. gen. and sp., the agelacrinitid Deltadiscus superbus, n. gen. and sp., and the pyrgocystinid lebetodiscids Archaepyrgus anitae, n. gen. and sp., and Fanulodiscus crystalensis, n. gen. and sp. Several unassigned edrioasterids are also described that provide the first information concerning edrioasterid ontogeny. All are more similar to Middle Ordovician relatives than to those of the Middle to Late Cambrian, even though the faunas are approximately equally spaced by age. The new edrioasteroids adhered to firm substrates, including hardgrounds, mounds, or bioclastic debris. Lampteroblastus has an elongate bud-shaped theca with short ambulacra and triangular deltoids that resemble and could be homologous with those of cyathocystid edrioasteroids. Thecal plates of Lampteroblastus also have heavy ridges reminiscent of certain camerate crinoids. Archaepyrgus, n. gen., and Fanulodiscus, n. gen., provide the first detailed pyrgocystinid morphology; unique aspects of their construction include the presence of lateral hood plates and loss of ambulacral floor plates. Deltadiscus has extremely narrow ambulacra and a short elongation zone Includes bibliographical references (p. 38-41) Five new edrioasteroid genera from the Early and Middle Ordovician of western Utah greatly enlarge the record of this scarce echinoderm class during its early diversification. Data from a diversity of taxa were used to generate a new phylogeny and classification of the edrioasteroids. New and reinterpreted morphology, particularly for the aboral surface, is introduced. Forty-two characters were scored for 15 taxa and subjected to a PAUP 3.0 parsimony analysis. This analysis identified two major edrioasteroid clades with subsequent subbranchings. One major clade is the isorophids, whose ancestry can be traced to the Early Cambrian; the second major clade includes edrioasterids and the former class Edrioblastoidea, dating to the Middle Cambrian. Edrioblastoids, rhenopyrgids, and cyathocystids mapped as specialized branches of the edrioasterid clade, while pyrgocystinids mapped as highly derived lebetodiscid isorophids. The cladistic analysis supported our hypothesis for analogous thecal elongation structures among edrioasterids and isorophids. -- The fauna contains the edrioasterid Paredriophus elongatus, n. gen. and sp., the edrioblastoid Lampteroblastus hintzei, n. gen. and sp., the agelacrinitid Deltadiscus superbus, n. gen. and sp., and the pyrgocystinid lebetodiscids Archaepyrgus anitae, n. gen. and sp., and Fanulodiscus crystalensis, n. gen. and sp. Several unassigned edrioasterids are also described that provide the first information concerning edrioasterid ontogeny. All are more similar to Middle Ordovician relatives than to those of the Middle to Late Cambrian, even though the faunas are approximately equally spaced by age. The new edrioasteroids adhered to firm substrates, including hardgrounds, mounds, or bioclastic debris. Lampteroblastus has an elongate bud-shaped theca with short ambulacra and triangular deltoids that resemble and could be homologous with those of cyathocystid edrioasteroids. Thecal plates of Lampteroblastus also have heavy ridges reminiscent of certain camerate crinoids. Archaepyrgus, n. gen., and Fanulodiscus, n. gen., provide the first detailed pyrgocystinid morphology; unique aspects of their construction include the presence of lateral hood plates and loss of ambulacral floor plates. Deltadiscus has extremely narrow ambulacra and a short elongation zone 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 (1966-1978). Fieldiana Geology New Series No. 1 began June 29, 1979