<P> The simpler structure is found in jawless fish, in which the cranium is represented by a trough - like basket of cartilaginous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril . Distinctively, these fish have no jaws . </P> <P> Cartilaginous fish, such as sharks, also have simple, and presumably primitive, skull structures . The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle . The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs . Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear . Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra . There are, in addition, at various points throughout the cranium, smaller foramina for the cranial nerves . The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper . </P> <P> In the ray - finned fishes, there has also been considerable modification from the primitive pattern . The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience . Other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little, if any bone in between them . The upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium . </P> <P> Although the skulls of fossil lobe - finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes . The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods . The upper jaw is formed from the pterygoids and vomers alone, all of which bear teeth . Much of the skull is formed from cartilage, and its overall structure is reduced . </P>

Which structure is a covering for the gill chamber in ray-finned fishes