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An ocellus is a porefield that has a distinct hyaline rim between its pores and the areolae. Plural is ocelli. Latin for "small eye".
Ocelli are found in polar centric genera such as Pleurosira and Eupodiscus.
Porefield Types
apical porefield - separated from valve striae by a narrow hyaline area; occurs at valve apex of some araphid (e.g. Diatoma) and some raphid genera (e.g. Cymbella)
ocellulimbus - porefield set into the apex of a valve (e.g. Pseudostaurosira)
ocellus - porefield with a distinct hyaline rim (e.g. Pleurosira)
pseudocellus - porefield lacking demarcation and with pores decreasing in size (e.g. Terpsinoë)
See ocellulimbus, porefield, and pseudocellus for more detail.