The mantle-edge gland produces the highly tanned, densely fibrous periostracum, and the cubocolumnar cells contribute to the deposition of the less highly tanned matrix of the inner shell layers. The mantle-edge gland gives positive reactions for dopa oxidase and peroxidase, but does not contain melanin. The cubocolumnar cells reveal a positive reaction for dopa oxidase, possess numerous melanin granules, and exhibit a negative reaction for peroxidase. It is suggested that quinones may contribute to the process of tanning and hardening of the structural proteins of the shell. The dopa oxidase reaction within the cubocolumnar cells indicates the presence of tyrosine and suggests that these cells are capable of producing quinones to color and harden the protein component of the inner shell layers. It also explains the abundance of melanin granules within these cells. Within the mantle-edge gland, it is suggested that the peroxidase inhibits the formation of melanin from dopa quinone, and peroxidase, by accentuating quinone production, may cause further hardening of the periostracum.