The hyobranchial skeleton of Proteus anguinus develops 3 branchial arches, whereas the vast majority of urodela undergoing metamorphosis develop 4 branchial arches. More characteristic is the fact that the ossification of the apparatus begins in young animals and continues into the adult stages, whereas ossification usually takes place essentially after metamorphosis. Retention of 3 ossified branchial arches by adult P. anguinus appears to be neither a larval characteristic nor a neoformation; examples of Amphiumidae and urodela fossils would support this. The hyobranchial skeleton of Necturus consists also of 3 branchial arches. The squat form, the individualised hypohyal, the very reduced ossification and denticulation of the ceratobranchials are, however, characteristic comparable to those of the neotenic axolotl. These characteristics indicate that Necturus has diverged further from European perennibranchiate fossils than Proteus.