Euryalida

Asteronychidae, Gorgonocephalidae

- RS with overlapping scales, easily fragmenting, flat articular structures

Gorgonocephalidae

- adGP bar-like, with bulbous distal end, articular structures large and flat

- abGP articulates proximal to bulbous part or may be fused to adGP

- madreporite inner side a meshwork of lamellae and pores