Shelled gastropods living on intertidal open rocky surfaces exhibit various shell features which can be interpreted as adaptations against such shell-breaking predators as fishes, spiny lobsters, and crabs. These anti-predatory devices include strong external shell sculpture, elongate or dentate apertures and low spires1. Not only are these characteristics more common among tropical species compared with ecologically similar cold-water forms, but within the tropics they are more prevalent and better developed in the Pacific and Indian Oceans than in the Atlantic. Any or all of three conditions must obtain in areas with a high incidence and strong development of anti-predatory armour compared with areas where such armour is reduced: the prey are stronger, that is, the criteria needed by the prey to avoid being crushed are more stringent; the predators are stronger; and shell-breaking predators account for relatively greater mortality. Here I report data confirming the first two of these possibilities