subopercular and opercular. [Illustration: From Wiedersheim, _Verg. Anat. der Wirbeltiere_, by permission of Gustav Fischer. FIG. 25.--Complete Skull of Salmon from left side. art, Articular. op, Opercular. branchiost, Branchiostegal. pal, Palatine. dent, Dentary. par, Parietal. epiot, Epiotic. pmx, Premaxilla. eth, Dermal ethmoid. preop, Preopercular. fr, Frontal. pt, Pterygoid. hyom, Hyomandibular. pter, Pterotic. intop, Interopercular. Quad, Quadrate. Jug, Jugal. socc, Supraoccipital. mpt, Mesopterygoid. sphot, Sphenotic. mtpt, Metapterygoid. subop, Subopercular. mx, Maxilla. sympl, Symplectic. nas, Nasal. Zunge, Tongue.] In addition to the bones already enumerated there is present a ring of circumorbital bones, a preopercular, behind and external to the hyomandibular and quadrate, and squamosal, external to the hinder end of the auditory capsule. In the salmon, pike, and various other Teleosts, extensive regions of the chondrocranium persist in the adult, while in others (e.g. the cod) the replacement by bone is practically complete. Bony elements may be developed in addition to those noticed in the salmon. In the sturgeon the chondrocranium is ensheathed by numerous membrane bones, but cartilage bones are absent. In the Crossopterygians[35] the chondrocranium persists to a great extent in the adult, but portions of it are replaced by cartilage bones--the most interesting being a large sphenethmoid like that of the frog. Numerous membrane bones cover the chondrocranium externally. In the Dipneusti[36] the chondrocranium is strengthened in the adult by numerous bones. One of the most characteristic is the great palatopterygoid bone which develops very early by the spreading of ossification backwards from the tooth bases, and whose early development probably accounts for the non-development of the palatopterygoid cartilage. _Appendicular Skeleton._--The primitive pectoral girdle, which in the Dipneusti is strengthened by a sheath of bone, becomes in the Teleostomes reduced in size (small scapula and coracoid bones) and replaced functionally by a secondary shoulder girdle formed of superficially placed membrane bones (supraclavicular and cleithrum or "clavicle," with, in addition in certain cases, an infraclavicular and one or two postclavicular elements), and connected at its dorsal end with the skull by a post-temporal bone. The pelvic girdle is in Teleostomes completely absent as a rule. The skeleton of the free limb undergoes ossification to a less or greater extent in the Teleostomes.