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SUMMARY A group of typical bony rays associated with a bony mass representing fused radial bones was found in the middle of the adipose fin of an otherwise normal, fully‐grown Synodontis membranaceus. A discussion of the evolution of the adipose fin leads to the conclusion that this structure was probably not derived from an ordinary rayed fin. The present abnormality cannot therefore be regarded as a reversion to an ancestral condition. Bony rays and spines occur normally in the second dorsal fin of some siluroids. They differ from the rays in the present specimen both in their position in the fin and in having no accompanying radial bones. It is therefore concluded that the factors which led to the production of this abnormality are not closely related to those which brought about the evolution of these other bony structures. It is suggested that the abnormality represents a type of homöosis not previously recorded in vertebrates.

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