Quondam Gill‐Covers

On the basis of its positional relations to the neuromast and branchial‐arch systems, a greater part of the cephalic exoskeleton in teleostome fishes is suggested to have evolved from dermal elements which once provided support for gill‐covers. More precisely, the following results are arrived at: parts of the decking of the ethmoid region of the endocranium stem from the branchiostegic exoskeleton of the terminal, or first, branchial unit; the external cheek‐plates are made up of modified branchiostegic components of the premandibular (second), mandibular (third), and hyoid (fourth) branchial units; the lower jaws include inferior branchiostegic elements of the mandibular and hyoid branchial units; those branchiostegic elements of the hyoid branchial unit which remained free of the external cheek‐plates and the lower jaws gave origin to the exoskeletal support of the hyoidean opercula as well as to the two rows of submandibular bones.