A superorder of the subclass Euechinoidea comprising primitive irregular sea urchins (Echinoidea) that have simple, undifferentiated tube feet and tuberculation and that retain a high degree of radial symmetry. They have a fully formed functioning lantern when juvenile, and they retain it throughout life. Eognathostomata comprises two orders, Pygasteroida and Holectypoida. Pygasteroids have a large, centrally positioned mouth; the apical disc has only four genital plates, the posterior plate having been lost. The periproct is aboral and lies in the posterior interambulacrum, either abutting the posterior margin of the apical disc (Pygasteridae) or separated by a relatively small number of interambulacral plates (Anorthopygidae). Holectypoids resemble pygasteroids, except that their periproct lies on the oral surface close to the peristome. They also have a compact apical disc in which there are five genital plates; the posterior genital plate may or may not be perforated by a gonopore.