rosette [paleont] . (a) A delicate calcareous plate formed of metamorphosed basal plates, centrally located within the radial pentagon in some free-swimming crinoids. (b) The group of five petal-shaped ambulacra on certain echinoids. (c) A cluster of parts in circular form; e.g. a discoaster. (d) A flower-shaped button within a hexagonal pore frame in a radiolarian skeleton. (e) The cellular part of a septula in cheilostome and ctenostome bryozoans.