Vertical blades that sit inside the corallite cup are called septa (singular is septum). The septa are separated from skeletal structures outside the corallite, by the corallite wall. The septa are often distinctive and may be used to differentiate between genera.
Septal spacing and size
Septa are arranged in a variety of plans and species level identification may be aided by examining these arrangements. However genera level identification requires a basic understanding of predominant patterns. Septa may be regularly, irregularly, closely or widely spaced. Where they are very regularly spaced over the wall of a meandroid colony they are described as zipper-like (e.g. Leptoria, p. 8 of the Coral Finder), or the corallite may be described as possessing regular ribs (e.g. Pachyseris, p. 7 of the Coral Finder).
In addition septa may be fine, thick or alternate in size (e.g. Montastrea, p.11 of the Coral Finder). Where the septa extend above the corallite they are called exsert, (e.g. Galaxea, p. 5 of the Coral Finder).
A key feature of septa are the teeth lining the upper margin; once again these are often distinctive to particular genera. Teeth may range from needle-like to spiky or blade-like. In addition the pattern can be regular orragged. For example, Lobophyllia has spiky septal teeth (p.6 of the Coral Finder), whereas those of Cycloseris are fine and regular (p. 24 of the Coral Finder).