Having, or approximating, the shape of a rhombus. A rhombus is a parallelogram with all four sides of the same length. Used to describe the shape of a valve (example, Staurosirella rhomboides), a central area, or the girdle view of a frustule (example, Eunotia rhomboidea).

Alternative Terms

Brachysira Arctoborealis Guide
Image Credit: Loren Bahls
Brachysira arctoborealis is considered to have a rhomboid valve outline.