Glossary

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The glossary provided here is largely based on “Seashells of Southern Florida: Bivalves,” by Paula M. Mikkelsen & Rüdiger Bieler, Princeton University Press. Production of this glossary was funded in part by the National Science Foundation’s Partnerships in Enhancing Expertise in Taxonomy [PEET] Program, NSF-DEB-9978119, “Bivalves - Research, Training, Electronic Dissemination of Data” (1999-2008) to Bieler and Mikkelsen.

U

UMBO (pl. UMBOS or UMBONES)
UMBO (pl. UMBOS or UMBONES) fig.Rounded or pointed extremity of a bivalve shell, often projecting above the HINGE line, that reflects the early growth stage (= oldest part of the shell) and includes the PRODISSOCONCH and adjacent convex area; also called beak (some authors treat beak and umbo as synonymous, but with some Bivalvia the umbos are above the beak) (figure: Dosinia discus).
UMBONAL CAVITY
Space inside the valve within the UMBO and under the HINGE PLATE.
UMBONAL CRACK
Crack in the shell of Periplomatidae that occurs naturally at the umbone of each valve (figure: Periploma margaritaceum).
UMBONAL REFLECTION
UMBONAL REFLECTION fig.Anterodorsal valve margin that is turned up and over the UMBO; also called flange or shield; occurring in such families as Pholadidae and Teredinidae (figure: Pholas campechiensis).
UMBONAL RIDGE
Prominent angled or rounded feature on the external shell surface that begins at the UMBO and runs obliquely posteriorly to the ventral border.
UNDULATE
Smoothly wavy, usually referring to a shell or MANTLE margin.