| Term | Definition |
| SUBTRIGONAL |
Not quite TRIGONAL, but nearly so. See other bivalve shapes. Photo: Nucula proxima Say, 1822 |
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| TRIGONAL |
Having three sides and angles; triangular in outline; see also: TRIANGULATE; DELTOID; DONACIFORM. See other bivalve shapes. Photo: Parastarte triquetra (Conrad, 1846) |
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| TRIGONALLY SUBOVAL | Having features of being triangular and somewhat OVAL in shape. See other bivalve shapes. | |
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