Bell elongate-cuboid, glassy-transparent in life, somewhat rigid, with flat top; reported to 220 mm long; length: breadth ratio possibly increasing with size. Exumbrella, pedalia and velarium typically, perhaps always, covered with nematocyst warts. Velarium with ca. 10 much-branched but not anastomosing canals in each quadrant. Pedalia 4, length approx. 2/5 of bell height, scalpel-shaped, sharp-edged. Rhopalia 4, complex; entrance to niche a horizontal slit. Tentacles when contracted approx. as long as bell height, with regularly spaced nematocyst bands; extending to ca. 3 m. Stomach occupying up to 1/3 length of bell; gastric cirri in 4 vertical interradial groups (in radii of bell corners). One half-gonad on each side of radial sinus; lamellar with frilled edges, when mature each extending to meet next.
Coloration: Bell glassy-transparent; exumbrellar warts translucent, spaces between translucence forming characteristic lace-pattern; tentacles milky-yellow; rhopalial photoreceptors brown.
Dangerous, sting painful but not lethal.
The somewhat similar Carybdea has no exumbrellar warts (except perhaps when small) and a T-shaped entrance to rhopalar niche. Carybdea atlantica (Barattini and Ureta, 1960) may be conspecific with Tamoya haplonema.
(Mainly after Mayer, 1910, and Kramp, 1961).