Umbo (bivalve)

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Diagram of the left valve of a bivalve resembling a venerid

A side view of a whole shell of Cucullaea labiata (Lightfoot, 1786), the umbones are at the top of the image

Diagram of a bivalve shell viewed from the dorsal side: 1:Plane of symmetry
2:Growth lines
3:Ligament
4:Umbo

The umbo, sometimes more informally called a "beak", is an (often somewhat protruding) part of each valve of the shell of a bivalve mollusk. The umbo formed while the animal was a juvenile, and radial growth has subsequently proceeded around that area. The umbo (plural umbones or umbos) are situated above the hinge line. In those bivalves where the umbones do not protrude, as is the case for example in some mussels, the umbones can nonetheless usually be readily identified by examining the concentric growth lines of the shell.^[1]^[2]

At all developmental stages, regardless of size, the shape and positioning of the umbones can be useful for identification purposes: the umbo can vary a great deal in shape and size, and these variations are sometimes species specific.^[3]

References

v t e Bivalve anatomy

Shell	Callus Hinge line Hinge teeth Ligament Lira Nacre Pallial line Pallial sinus Periostracum Prodissoconch Resilifer Resilium Sculpture Umbo Valve

Other hard parts	Byssus Pearl Sea silk

Soft parts	Adductor muscles Ctenidium Gastric shield Mantle Nephridium Siphon

Other	Abductin Glochidium Pseudofeces Trochophore Veliger

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