The Pseudodeltidium of the Strophomenid Brachiopods

Abstract

The term pseudodeltidium was proposed by Bronn (1862, p. 242) for the shell-plate covering the delthyrium, or triangular opening between the teeth, in the ventral cardinal area of those brachiopods now assigned to the order Protremata. It is typically developed among the “family” Strophomenidae, of which it is a diagnostic feature, as a convex arch overhanging the interior of the ventral valve (Text-fig. 1A). In the embryo, the pseudodeltidium is secreted as a single unit when the shell is first formed, and is therefore not homologous with the deltidium (von Buch, 1834, p. 15) of the order Telotremata, since this is formed in the post-embryonic period as two separate plates which may subsequently coalesce (Beecher, 1892)