Theumbo (pl. umbones orumbos) is the vaguely defined, often most prominent, highest part of eachvalve of theshell of abivalve orunivalvemollusc. It usually contains the valve'sbeak, the oldest point of the valve, and its degree of prominence and position relative to thehinge line are sometimes helpful in distinguishing bivalvetaxa.[1] The umbo forms while the animal is a juvenile, and radial growth subsequently proceeds around that area. The umbo is situated above the hinge line.
In those bivalves where the umbones do not protrude, as is the case for example in somemussels, the umbones can nonetheless usually be readily identified by examining the concentric growth lines of the shell.[2][3]
Umbo is also in use in anatomic descriptions ofbrachiopods, for the origin of growth of the valves.
