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.