Key to the Corals of Bermuda

20. Cups 2.5-3.5 mm in diameter, septa with inner margins almost vertical, producing steep-sided narrow pits...Siderastrea radians

Cups 4-5 mm in diameter, inner edges of septa sloping about 45 degrees, producing shallow pits...Siderastrea siderea

(Some gradation in cup characteristics may occur on a colony of Siderastrea, suggesting that these two species may possibly belong to a single highly-variable form).