The anatomy of the wrist of the Japanese giant flying squirrel Petaurista leucogenys was investigated, and, on the basis of myological data, a hypothesis was formed concerning the origin of the long accessory styliform cartilage supporting the flying membrane. The styliform cartilage in the forelimb of P. leucogenys lies along the ulnar side; its length is almost equivalent to that of the radioulna. During the present study, it was recognised that both the flexor carpi ulnaris muscle and the flexor retinaculum attach to the proximal end of the styliform cartilage. The results of our anatomical studies suggest that in P. leucogenys, the styliform cartilage is essentially homologous to the pisiform bone in other mammals and is presumed to have originally been derived from a sesamoid bone.

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