A small class of "arm"- and brachiole-bearing, stemmed echinoderms in the subphylum Crinozoa, based on five genera known from the Middle Ordo-vician to Late Silurian of Europe and North America. Coronoids have a crested theca or body with well-developed pentameral symmetry and plate arrangement very similar to that found in blastoids (see illus.). Skeletal plates include three basals (two large and one smaller), five radials extending up to form the crests and bearing central notches for the ambulacra, five small plates supporting the arms, and four regular deltoids plus two anal deltoids in the fifth position around the mouth. The mouth is central on top of the theca and has five ambulacral grooves radiating from it. A coiled arm is attached to a mounting plate at the end of each ambulacral groove; each arm has biserial plating with smaller biserial branches (= brachioles) on both sides. The projecting crests have internal canals that connected with the body cavity and apparently served as respiratory structures. A thin stem supported the theca above the sea floor, allowing coronoids to live as attached, low- to medium-level suspension feeders. Coronoids had previously been assigned to the blastoids, eocrinoids, or crinoids by different researchers, but they have been elevated in rank to a separate class. Coronoids appear to be most closely related to the Blastoidea and may have been the ancestors of this class.