An Illustrated and Cross-Referenced Glossary of
Malacological and Conchological Terms

by Paul S. Mikkelsen

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Term Definition

CIRCUMBOREAL   Distributed around the world, in the northern BOREAL region. See also: PROVINCE.

  Region of VALVE (in bivalves) surrounding point of maximum curvature of longitudinal DORSAL profile and extending to the BEAK, when not coinciding with it (many authors treat "beak" and "umbo" as synonymous, but with most shells, two distinct terms are needed). See illustration: Composite Bivalve. See also: NATES; PRODISSOCONCH.

See: Bivalve terminology, Anatomy of bivalves by Marlo Krisberg.

UMBONAL   Pertaining to the UMBONES.

UMBONAL ANGLE   In pectinoid shells, the angle of divergence of UMBONAL FOLDS. In other shells, approximate angle of divergence of POSTERODORSAL and ANTERODORSAL parts of longitudinal profile.

UMBONAL CAVITY   Part of the interior of the valve (of bivalves) that lies within the UMBO and under the HINGE PLATE (when present).

UMBONAL DEPRESSION   Small depression at the tip of the UMBO in some Arcacea.

UMBONAL FOLD   In pectinoid shells, the ridge originating at the UMBO and setting off the body of the shell from the AURICLE.

UMBONAL POLE   Point of maximum curvature of longitudinal dorsal profile of valve. See also: UMBO.

UMBONAL RIDGE   A radial ridge that sets off the posterior from the central and anterior slopes of a bivalve shell. See also: UMBO.

UMBONAL SLOPE   The line of greatest CONVEXITY which can be traced from the BEAK to the anterior or posterior ventral corners of the valve of bivalves; see also: CORSELET.

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