Tilesius, 1802
(Diagnosis corresponding to that of the genus Thetys, since only a single species has been recognized).
Solitary zooids: up to 230 mm long, excluding projections (Thetys vagina). Test firm and thick, with large spines scattered irregularly on its surface; a prominent posterior projection on each side of the atrial opening. Twenty weakly developed muscles (16-22 according to Thompson, 1948), all of them interrupted dorsally and ventrally.
Aggregate zooids: up to 100 mm long (Thetys vagina 1). Test firm and thick, with longitudinal rows of large spines. Five body muscles interrupted dorsally and ventrally. MI to MIII converging dorsally but not contiguous; MV bifurcate. Three or 4 embryos between MIV and MV.