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Abstract
A number of extant Aulacodiscus species collected in marine coastal habitats in California, Florida, Australia and Africa were examined with light and scanning electron microscopy. Information on the sequence of development in valves of this genus was inferred by the structures seen in immature specimens. Tubes grow outwardly from evenly spaced openings at the margins of the valve in the same position where internal flaps of silica initiate curved slits. Aulacodiscus scaber and A. petersii are similar in pattern of development and both have complex internal structures within their tubes. A. africanus and A. kittonii are another natural group with slitted external tube openings. A. argus is similar to the fossil species A. rogersii and A. grevilleanus; all have clusters of very small loculi in depressions with surrounding broad ridges and bullulae and can be presumed to have formed in like manner. The formation of the bubble-like bullulae was followed in A. oregonus and A. argus. Some features, visible in early valves, are obscured by later silica deposition.