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But in this case, scientists discovered a giant structure built entirely out of one species of coral, known as Pavona clavus.

The creatures aren't graceful swimmers; they’re often seen “waggling their large dorsal and anal fins to move and steering with their clavus,” according to National Geographic.

Somewhat magnified. sc, scutellum; co, cl, m, corium, clavus and membrane of forewing.

This much may be said, however: that the pain is rarely or never seated in one parietal region, as is frequently the case with migraine and with clavus.

Hysterical patients describe a sensation as if a nail were being driven into the forehead—the so-called clavus hystericus.