Genus Trachythyone Studer, 1876

Diagnosis (after Panning 1964). Usually slender, curved or U-shaped species, somewhat pentagonal in cross section. Tube feet restricted to the ambulacra in usually single or double rows, sometimes also scattered in the interambulacra. Tentacles 10, two ventral most reduced. Skin often thin but rigid. Calcareous ring simple, without posterior bifurcations to the radials plates; radial and interradial plates with long, narrow, anterior projections; radial plates with a distinct notch in posterior margin. Deposits of the body wall a superficial layer of small baskets, often reduced to cruciform plates, and an inner layer of large, mostly smooth, sometimes knobbed and imbricating multilocular plates.