Size up to 2¼ inches. Shell ovoid with an ovoid opening; greatly compressed and almost completely imbedded in the skin of the whale. Body-chamber surrounded by about thirty rays which project above the surface of the skin. Cover-plates, usually two, almost invisible. Opercular opening covered by a strong membrane with a central hood through which the cirri are protruded.
Colour of the shell, white where eroded; opercular membrane, sulphur yellow.
On California grey whale Eschrichtius glaucus (Cope) which is confined to the North Pacific Ocean.
Cryptolepas rachianectiis almost wholly embedded in the whale's skin, usually on the head and under-parts of flippers and fluke. "It is excessively variable in size, proportion of diameter to height, and shape of the ribs or lamellae, which may be short, with a rounded (semicircular) outline, or long, and triangular in outline. The ribs may have many or very few branches. In young specimens the shape is that of a thick disk with rounded periphery, and the folds are simple. At all stages their free edges are beautifully crenulated." (Pilsbry.). The amphipod Cyamus scammoni Lutken is often found on the opercular membrane or in the shallow cavities of the eroded radii.