If corallites are arranged in valleys (see above for more info) and neighbouring valleys share ridges the colony is called meandroid. The genus Symphyllia (p. 9 of the Coral Finder) may have distinct shared ridges.
Where corallites are arranged in valleys (see above for more info) and neighbouring valleys have separate walls, the colony is described as flabello-meandroid. The genus Plerogyra (p. 27 of the Coral Finder) forms flabello-meandroid colonies. This is one of the ways in which Plerogyra can be differentiated from the similar genus Physogyra, which is meandroid in form.
(Photo of Plerogyra)
The term meandering is not traditionally used to describe a coral life form, but it is a very quick and easy way to differentiate between sets of genera, and therefore has been used as a key grouping in the Coral Finder. Meandering corals have distinctive colonies covered in valleys separated by ridges or walls (e.g. Leptoria, p.8 of the Coral Finder). If these meanders are present but the valleys are short, the colony may be described as weakly meandering (e.g. Goniastrea, p. 8 of the Coral Finder). For more information on meandering forms check out the Corallite Arrangement article.
(Photos of meandering to weakly meandering forms)