White form of A. proxima (left) and yellowish form of O. muricata (right). Both from Orkney, November 1975. In England and Wales the most frequent colours of the two species are exchanged. O. muricata has tubercles with broad, flat or gently rounded tops except the smallest tubercles at the mantle edge. A. proxima has tubercles with a rounded point and varying inflation. Dorsal spicules of mantle usually more obvious on O. muricata than on A. proxima.