13. Notes on the Chondrooranium and Branchial Skeleton of Salmo

In the adult Salmo the lower end of the hyomandibular and the upper end of the synxplectic are well ossified, but divided by a cartilage. A long palato‐quadrate cartilage is present and has a well‐developed orbital process reaching the inner edge of the mesopterygoid. In some specimens of Salmo trutta in the region of the orbital process are found two ossifications lying upon the mesopterygoid. The branchiostegals in Salmo are asymmetrical, the left being better developed and often exceeding the right in number. There are two copulas in the adult Salmo; the anterior is related to the hyoid arch and the first two branchial arches; the posterior copula is related to the three posterior branchial arches. The third branchial arch has a complicated articulation with the posterior copula, i. e., by means of the anterior ends of the cerato‐branchials 3 and the posterior ends of the hypo‐branchials 3. There are two supra‐copularies which in adult Salmo are usually fused with the basi‐branchial 3, but in some large specimens of Salmo trutta caspius the supra‐copularies are found to be independent of the proper elements of the copula. There are two pairs of dentigerous pharyngeal plates which even in the adult Salmo do not fuse with the proper elements of the branchial arches. Separate pharyngeal teeth are usually present on the pharyngo‐branchial 3. There are double rows of gill‐rakers (outer and inner rows) on the first four branchial arches, and one outer row on the fifth arch. The inner row of the first arch is often feebly developed, but always present. The roof of the ethmoid cavity (anterior myodome) is present in adult Salmo, but is differently developed in different species. It is comparatively strong in S. salar, thin in S. trutta, and very thin and pierced with holes in S. trutta caspius. Small pre‐ethmoidal cavities or ducts in the cartilage of the rostrum are also present. No true mesethmoid bone is to be found in S. salar, S. irideus, and the freshwater forms of S. trutta, but in the migratory form of S. trutta more or less developed ossifications of the ethmoid cartilage were found in some of the specimens investigated. In S. trutta caspius such ossifications were found more frequently and in fuller development. In the genus Stenodus of the same family a well‐developed true mesethmoid bone seems to be constantly present. In no case in Salmo or in Stenodus does the supra‐ethmoid fuse with the true mesethmoid bone. In one of the specimens of Salmo salar examined no nasal was present on the right side, and the mucous canals, which normally would be present in the nasal bone, were enclosed in the right side of the supra‐ethmoid. Some of the characters of the skull of Salmo have undoubted taxonomic significance, and, anticipating the account of my investigations on the group of Oncorhynchus, these characters are summarised in the following table.