Extinct Pelmatozoa, either sessile, or furnished with a short stalk. The body is ovate or globular, and covered with polygonal plates rarely arranged with regularity. The stem-joints are round and ring-like, the median canal being large. It is pointed below, and the joints are sometimes telescoped one into the other. The mouth is elevated and central, and five ambulacral grooves may be traced radiating from it in many instances. A tube covered by plates probably represents the anal tube, and a third aperture, sometimes present, a genital or ovarian opening. Arms, two to five in number, are generally present, but are feeble and unbranched. They may carry pinnulae, and the ambulacral grooves may be protected by covering plates. Elevations pierced by double pores ( b= water-pores ?) are developed on more or fewer plates in many species. In others there are grooves pierced by pores which form a rhombic figure, one part of the figure being on one plate, the other on an adjoining plate. These pectinated rhombs are sometimes few in number, isolated, and devoid of pores; and the two halves of the rhomb are occasionally separate and distinct.
The Cystoidea appear in the Lower Silurian, reach a maximum of development in the Upper Silurian, and die out in the Carboniferous strata in which they occur sparingly.