The vertical distribution of the Rhombognathinae in the intertidal zone and hence their tolerance to wave action, is correlated with the structure of the ambulacrum. Lateral claw structure, and in particular the development of combs, is an important factor, with heavy multidentate combs providing most grip, and therefore greater tolerance to. wave action. The median claw is of secondary importance because it provides Jess grip than the combs which present a greater number of points of contact with the substratum.