Simnia spelta grows to a length of about . The shell is roughly egg or cigar-shaped with a flattened base. It is strong, thick, smooth and glossy, with inrolled edges at the sides of the long aperture on the underside. The colouring varies according to the species of
gorgonian on which it is living. The background colour of the shell is white, yellow, pink or orange, often with white spots or dark spots, marbling or streaks, providing
camouflage. As is commonly the case in
Simnia species, the colour of all the visible soft parts normally matches the colour of the gorgonian on which the snail is grazing, as the pigment from the gorgonian is retained in the
mantle of the mollusk. ==Distribution==