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Aulacodiscus species can be differentiated by their tube openings, not only in the number on the valve face near the margin but also in terms of the shape and whether the tubes have external flanges or distinct internal structures. Tube openings are positioned on the valve and are indicative of symmetry. Tube opening formation occurs as valve formation proceeds. Externally flanged with tongued and ridged internal tube structures are present in circular tubed A. scaber and A. petersii in contrast to A. africanus and A. kittonii with slitted tube openings with no distinct internal structures. The fossil species, Aulacodiscus rogersii, has a flanged circular tube opening as does A. oregonus. Tube openings are formed near the valve margin, and their number and position on the valve margin are indicators of valve formation [2.144].
Areolae are loculate or poroid in Aulacodiscus, and the areas without pores that radiate from the central annulus form the rays of the valve face that extend to the tube openings. Occluded areolae occur as well at the valve margin. Cribra form from internal struts of the areolae, and as bullulae develop in early tube formation, they are covered by silica at the completion of tube formation. Cribra
