Extinct Pelmatozoa with a short stem, an ovate body, and five-rayed ambulacral area. The calyx consists of three basals, five radials and five (interradial) orals. Each radial is forked, inclosing a sinus, in which is lodged an ambulacral area. The orals, or 'deltoid pieces,' are attached to the ends of two adjoining prongs belonging to two different radials or *fork pieces! The mouth is central. The ambulacral area is formed by five lancet plates with median longitudinal grooves. Side plates rest upon each lancet plate laterally, and in some forms there is also a row of outer side plates. Jointed pinnules in a single or double row are attached to the outer edge of each ambulacral groove. The inner side plates are marked by grooves leading outwards, each one to a marginal pore. The pores lead to a cleft (hydrospire cleft) at the outer edge of the lancet plate, and the cleft in its turn to an underlying hydrospire canal, into which open a system of interradial lamellar tubes, the hydrospires, supposed to be respiratory in function. A single or double aperture lies at the apex of each oral or deltoid piece. These are the spwacles or efferent external apertures of the hydrospires. The anus is confluent with one of them.
The genital ducts probably opened into some portion of the hydrospires, and the spiracles in that case discharged the genital products. A circum-oral ring with five radiating tubes, one under each lancet piece, represents the water-vascular ring and ambulacral tubes.
In some American forms a vault of minute plates, without definite arrangement arches over the mouth and spiracles, except in Elecrinus, where there is an oro-central and five orals. A series of covering plates may be continued down the ambulacral area concealing the groove. This arrangement is identical with what obtains in many Palaeo-crinoidea.
The Blastoidea appear later than the Crinoidea and Cystoidea in the Upper Silurian, and reach their greatest development in the Devonian and Carboniferous strata in which they die out.
Carpenter (P. H.) and Etheredge, A. N. H. (5), ix. 1882; Carpenter, op. cit, (5), xv. 1885.