The skulls of all species of Oncorhynchus which I have studied and examined (O. keta, O. gorbuscha, O. tschawytscha, O. kisutsch, and O. masu), although differing somewhat in each species, have similar general characteristic breeding changes which can be summarised as follows:-

A great part of the elements of the skull are more or less affected by the breeding changes. Those most affected are the ethmoid region of the chondrocranium and all the tooth-bearing bones.

The changes are greater in large specimens, and also in males than in females. The changes consist mainly in an enlargement of the affected parts, especially in length, and of the formation of new “breeding” teeth.

The breeding teeth of Oncorhynchus, unlike those of Salmo, have large ossified bases and are not set in sockets.

The teeth of the præmaxillary develop not only in the space covered by the bone but also in front of and behind it. The anterior of the tooth-bases from the right and left sides overlap the tip of the “rostrum” and join together in front of it, but no space is left between them and no hollow is developed by the anterior parts of the præmaxillaries in front of the “rostrum” In breeding males of Oncorhynchus the lengthened and curved “rostrum” and the præmaxillaries project noticeably beyond the anterior end of the lower jaw. At the anterior end of the maxillary, teeth with large bases are also developed and, uniting with the curved end of the maxillary, completely fill this space. Thus, the maxillary in breeding specimens of Oncorhynchus becomes straight.

Very large tooth-bases develop on the anterior end of the dentary. Uniting with the bone they form a kind of knob crowned with teeth. The anterior ends of both the dentaries grow closely together and no groove is formed between them.

Long needle-shaped teeth develop on the pharyngeal plates.

All species of Oncorhynchus breed only once.

Comparing these changes with those of Salmo, we see that they are markedly different. In Salmo salar the breeding teeth have no ossified bases and the teeth are set in sockets. The lengthening of the præmaxillary is due exclusively to the growth of the bone itself. The anterior ends of the præmaxillaries grow beyond the forked “rostrum” and a large hollow is formed between them. No teeth grow beyond the præmaxillary or at the inwardly bent anterior end of the maxillary. The anterior ends of the dentaries themselves grow dorsally and form an upturned knob. A large hook, consisting of connective tissue, appears on the front end of the lower jaw, its base being fastened in a deep groove between the tips of the dentaries. The teeth take no part in the formation of this hook and are never fastened to it. The upper jaw in Salmo never projects beyond the anterior end of the lower.

All the breeding changes in Salmo are recurrent and the breeding characters disappear after spawning.