Both A. proxima and O. muricata have club shaped tubercles with a narrow base. A. proxima tubercles (top) are ovoid with a rounded point and a not easily visible central core of spicules. O. muricata tubercles (bottom) are more spherical and have broad, flat or gently rounded tops and spicules near the surface resembling segments of an orange. Orkney November 1975 (left) and North Wales, March 2012.