In the _Holocephali_ part of the labyrinth is enclosed in the cartilage of the skull, another part being in the cranial cavity, as in Ganoids and Teleosteans. The membranous vestibulum is continued by a canal to a single opening in the roof of the skull, from which two smaller canals are continued to two small foramina in the skin covering the occipital region. In the _Teleosteans_ the sac which contains the otoliths lies on each side of the base of the cranial cavity and is often divided by a septum into two compartments of unequal size, each containing a firm and solid _otolith_; these bodies (Fig. 51), possess indented margins, frequently other impressions and grooves, in which nerves from the N. acusticus are lodged; they vary much in size and form, but in both respects show a remarkable constancy in the same kind of fishes. The vestibule is outwards in contact with the osseous side wall of the skull, inwards with the metencephalon and medulla oblongata; it contains another firm concretion, and opens by five foramina into the three semicircular canals. The terminations of the acustic nerve are distributed over the vestibular concretion and the ampulliform ends (Fig. 52 _p_) of the semicircular canals, without being continued into the latter, which are filled with fluid. The semicircular canals (Fig. 52 _g_), are sometimes lodged in the cranial bones, sometimes partly free in the cranial cavity. Many Teleostei have fontanelles in the roof of the skull, closed by skin or very thin bone only at the place where the auditory organ approaches the surface, by which means sonorous undulations must be conducted with greater ease to the ear. [Illustration: Fig. 51.--Otolith of Haddock (Gadus æglefinus). I. Outer, II. Inner aspect.] In many Teleostei a most remarkable relation obtains between the organ of hearing and the air-bladder. In the most simple form this connection is established in Percoids and the allied families, in which the two anterior horns of the air-bladder are attached to fontanelles of the occipital region of the skull, the vestibulum occupying the opposite side of the membrane by which the fontanelle is closed. The condition is similar, but more complicated in many Clupeoids. The anterior narrow end of the air-bladder is produced into a canal at the base of the skull, and divided into two very narrow branches, which again bifurcate and terminate in a globular swelling. An appendage of the vestibulum meets the anterior of these swellings, and comes into close contact with it. Besides, the two vestibules communicate with each other by a transverse canal, crossing the cranial cavity below the brain. [Illustration: Fig. 52.--Communication between auditory organ and air-bladder in the Carp. (After E. H. Weber.)