Shell is narrowly elongated and triangular in shape, as the animal's German name suggests. The umbo lies at the pointed anterior end. The shell is rounded at the posterior end, inflated, smooth, narrowly gaping ventrally, with a sharp mid-valve keel producing a flattened ventral surface. The shell is variably banded with dark-brown-and-cream "zebra" markings. Grows to about 2 in (5 cm) in length. Has an internal shelflike septum at the umbo for muscle attachment. Soft body with short, separated siphons; produces byssal threads that exit the shell at a ventral byssal gape.
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