Trachythyone

Studer, 1876

17 species

Ten tentacles. The calcareous ring simple, without forked tail. In the body wall the cups and smooth plates cover each other like scales in a few species. (Panning, 1949)

Curved, U-shaped, stunted to the fore. The hind end is often tail shape. The skin is mostly thin and rigid (stiff). The mouth does not have flaps; there are 10 tentacles, the 2 ventral ones are smaller. The calcareous ring is simple and without forked tail. The retractors are short. The feet are either: (1) in a double row in the radii; or (2) in single rows in the radii, a few of them going over to the interradii as well; or (3) distributed almost without pattern over the radii and interradii, ventrally somewhat closer together, dorsally, more sparse. The middle dorsal interradius can remain free (empty of feet). The feet in the base portions are banded (striped) with layers of calcareous deposits and not easily retractable. The calcareous deposits are surface positioned cups, some deeply hollow, some flat, some transformed into rosette shape, flat plates; in deeper position these are large, mostly smooth plates, which more of less cover each other at the edges or in a genuine roof tile pattern formation. (Panning, 1964)

Ten tentacles. Calcareous ring simple, without forked tail. In the body wall the cups and smooth plates cover each other like scales in a few species. (Pawson, 1971a)