bivalate

bivalate, bivallate

In military architecture, a pair of defensive ditches and earth embankments, usually concentric, that surround a mound or medieval fort.
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Sori on the tips of the upper ultimate segments; indusium bivalate on upper part, with simple, forked or 3-tufted, stalked, stellate hairs on lip margins; receptacle short, included, not protruding.