There remain, finally, the bones distinguishable on the upper surface of the skull; the largest, extending from the nasal cavities to the occipital, are the _frontal_ bones (1), which also form the upper margin of the orbit. The _postfrontals_ (4) are small bones placed on the supero-posterior angle of the orbit, and serving as the point from which the infraorbital ring is suspended. The _pre-frontals_ (2), also small, occupy the anterior margin of the orbit. A pair of small tubiform bones (20), the _turbinals_, occupy the foremost part of the snout, in front of the frontals, and are separated from each other by intervening cartilage. After removal of the gill-cover and mandibulary suspensorium, the hyoid arch, which encloses the branchial apparatus, and farther behind, the humeral arch are laid open to view (Fig. 25). These parts can be readily separated from the cranium proper. The _hyoid arch_ is suspended by a slender styliform bone, the _stylohyal_ (29), from the hyomandibulars; it consists of three segments, the _epihyal_ (37), _ceratohyal_ (38), which is the longest and strongest piece, and the _basihyal_, which is formed by two juxtaposed pieces (39, 40). Between the latter there is a median styliform ossicle (41), extending forwards into the substance of the tongue, called _glossohyal_ or _os linguale_; and below the junction of the two hyoid branches there is a vertical single bone (42), expanded along its lower edge, which, connected by ligament with the anterior extremity of the humeral arch, forms the _isthmus_ separating the two gill-openings. This bone is called the _urohyal_. Articulated or attached by ligaments to the epihyal and ceratohyal are a number of sword-shaped bones or rays (43), the _branchiostegals_, between which the branchiostegal membrane is extended.