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Stony ridges forming part of the corallites of corals
In corals, a septum (plural septa) is one of the radiating vertical plates lying within the corallite wall. Outside the corallite wall these plates are known as costae (singular costa). The septa may be thick, thin or vary in size. They may have teeth which range from needle-like to blade-like and are often characteristic of different genera.
References
Sprung, Julian (1999). Corals: A quick reference guide. Ricordea Publishing. pp. 220–223. ISBN1-883693-09-8.
"Septa". Coral Hub. Archived from the original on 2015-04-25. Retrieved 2015-04-22.