umbones

Umbones are the beak-like projections at the hinges of bivalve mollusk shells. Each valve has an umbo, typically located near the hinge line, and this region represents the oldest part of the shell.

The umbo is usually the highest point on the valve and from it the shell grows outward; growth increments radiate from the umbo as the animal increases in size. The size, shape, and position of the umbo vary among species, which makes umbones useful in shell identification.

In living and fossil shells, the umbo helps establish orientation of the shell and age estimates. The relative anterior and posterior directions are inferred from shell morphology, with the umbonal region commonly pointing toward the animal's anterior in many species.

In paleontology and malacology, umbones are important diagnostic features. Their development, morphology, and spacing of growth rings provide information about growth rates, life span, and evolutionary relationships among bivalves.

Umbones are distinct from other shell features such as the hinge teeth and the margin, and the term is used primarily in reference to bivalve anatomy.