Fine radial ridges that run from ventral margin to and across umbo, and are usually numerous on any given valve (e.g. Conchostraca) [Moore and McCormick, 1969].
(Order Diplostraca):
Fine radial ridges extending from ventral margin across umbo [McLaughlin, 1980].
Crustacea glossary. Natural History Museum of Los Angeles County. 2011.