The ocular sinuses of seven genera of ostracods (Trachyleberididae) were studied through the use of internal molds and thin and polished sections. The three-dimensional aspects of the ocular sinus in the genera Actinocythereis, Henryhowella, Malzella, Orionina, Puriana, and Radimella have not been studied before. Echinocythereis, previously studied, was included for comparison.

Morphology of the sinuses varies from the low moundlike form in some specimens of Henryhowella, to the long structure in Actinocythereis. With the exception of Henryhowella, all other genera have ocular sinuses with many features in common. These properties include a stalk-like appearance marked by a distinct constriction someplace along the length of the stalk. Most of the sinus is circular in cross section except the distal portion, which is expanded and irregular in shape, presumably to accommodate the portion of the eye with the lens cell, retinal cells, and rhabdomes. The surface of the distal portion is dominated by a concavity that is the complement of the convexity of the adjoining eyespot. Adjacent to the convexity is an anterior lobe or rim and usually a higher posterior rim Channels with diameters of 2 to 6 µm probably are passageways for axons. Ontogenetic development includes an early, low, cone-shaped sinus with a central concavity. Later juveniles in most taxa have a stalk, as do adults.

Despite overall similarities of the ocular sinuses, detailed examination indicates that genera and species can each be distinguished using this feature. Variations in size, shape, proportions, and secondary topography are diagnostic; thus, another character is available for taxonomic purposes.

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