L. Agassiz, 1862
The "cannonball jellyfish" of U.S.A. Bell slightly more than a hemisphere and slightly steeper-sided, solid, up to ca. 185 mm diameter; mouth-arms short, fused together, lacking clubs or filaments. Jelly thick, rigid. Exumbrellar surface smooth, with minute nematocyst warts. Marginal lappets up to ca. 14 per octant, short, rounded; rhopalar lappets up to 3 times longer, pointed. Rhopalia 8, in conspicuous pear-shaped pits on margin. Subumbrellar circular musculature interrupted over radial canals; radial musculature in 16 outward-pointing triangular patches, the bases against manubrium. Mouth-arms fused along most of length to form solid "manubrium", grooved externally, projecting slightly beyond perimeter; free ends bifurcating, flared; unusually in rhizostomes, a central mouth present permanently; basally on each mouth-arm two scythe-shaped scapulets (16 in all) bearing mouthlets. No clubs or filaments. Stomach occupying central 1/5 of bell; radial canals 16, connected by broad anastomosing network over most of bell; no ring canal.
Coloration greatly varied: exumbrella brown, cream, milky bluish to yellowish (lavender to pink reported); some specimens with patches and bands of color, especially a peripheral band; gastrodermis reported dull yellow; exumbrella surface may also be reticulated brown, especially densely in lower 1/3 of bell where also occur numerous white to yellowish spots; mouthlet regions brownish to pale pinkish-fawn.
(Partly after Mayer, 1910).