A small class of spindle-shaped, spirally pleated, primitive echinoderms in the subphylum Echinozoa, from the Early Cambrian (Nevadella Zone) in eastern California, western Nevada, and eastern British Columbia. Since their discovery in the early 1960s, three genera and six species have been described based on nearly 600 complete specimens, making helicoplacoids the most diverse and abundant echinoderm class known from the Early Cambrian. Most helicoplacoids have a spindle-shaped theca or body with diagonally spiraled pleats, each made up of three rows of flexibly sutured plates forming an interambulacral ridge. Smaller-plated ambulacra spiral around the theca every 7–13 pleats; the ambulacral plates cover a central food groove that has pores for tube feet on each side. The original authors inferred that the mouth was at the more rounded pole of the theca and that a single long ambulacrum branched once about one-third of the way down the theca. However, it has been argued that the mouth was located where the ambulacra split (see illus.), so that three ambulacral branches lead away from it, one spiraling partway down the theca and two spiraling up the theca a few pleats apart. No anal opening or pyramid has been found on any of the known specimens.