Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/660

642 ICHTHYOLOGY [OSTEOLOGY, than in higher vertebrates, and are separated from each other by the anterior prolongation of the supraoccipital. The squamosal (12) has been mentioned above in connexion with the mastoid. The prefrontal (2) and supraorbital are always small, and the latter is frequently absent. The lower surface of the skull is protected by the basisphenoid (parasphenoid) (6), a very long and narrow bone extending from the basioccipital beyond the brain-capsule to between the orbits, where it forms the support of the interorbital septum. Anteriorly it is connate with another long hammer-shaped bone (16), the vomer. Both these bones, especially the latter, may be armed with teeth. 3. Cartilage-Bones of the Alimentary Portion of the Visceral Skeleton of the /Skull. The suspensorium consists of three cartilage-bones, and affords a base for the opercular apparatus as well as a point of attachment to the hyoid, whilst in front it is connected with the pterygo-palatine arch (fig. 27). These are the hyomandibular (23), sym- FIG. 27. Lower view of Skull of Perch. plectic (31), and quadrate (26) ; they are connected by means of the metapterygoid (27) with the pterygoid (24) and entopterygoid (25), the foremost bone of the arch being the palatine (22). Of these bones the uppermost, the epitympanic or hyomandibular (23), is articulated by a double articulary head with the mastoid and posterior frontal. Another articulary head is destined for the opercular joint. The mesotympanic or symplectic (31) appears as a styliform prolongation of the lower part of the hyomandibular; it is cartilaginous in the young, but almost completely ossified in the adult. The position of this bone is noteworthy, because, directly inwards from its cartila ginous junction with the hyomandibular, there is situated the uppermost piece of the hyoid arch, the stylohyal. The next bone of the series is the pretympanic or metapterygoid (27), a flat bone forming a bridge towards the pterygoid, and not rarely absent in the subclass. Finally, the large triangular hypotympanic or quadrate (26) has a large condyle for the mandibular joint. The palatine arch proper is formed by three bones : the entopterygoid (25) is an oblong and thin bone attached to the inner border of the palatine and pterygoid, and increasing the surface of the bony roof of the mouth towards the median line ; it constitutes also the floor of the orbit ; the pterygoid (or os transversum) (24) starts from the quadrate, and is joined by suture to the palatine, which is toothed in the perch and many other fishes, and reaches to the vomer and anterior frontal. The piece of the mandible which articulates with the quadrate is the articulary bone (35), distinctly part of Meckel s cartilage ; it sends upwards a coronoid process to which the masticatory muscles, as well as a ligament from the maxillary, are attached; it also sends forwards a long-pointed process to be sheathed in a deep notch of the dentary piece. Frequently another por tion of cartilage below the articulary remains persistent, or is replaced by a separate membrane-bone, the angular. 4. Membrane-Bones of the Alimentary Portion of the Visceral Skeleton of the Skull. The suspensorium has one tegumentary bone attached to it, viz., the praeoperculum (30) ; it is but rarely absent, as for instance in Murcenophis; as it is quite a superficial bone, and frequently armed with spines (as in the perch), its form and configuration constitute an important item in the description of many fishes. The premaxillary (17) and maxillary (18) of the Teleostei appear to be also membrane-bones, although they are clearly analogous to the upper labial cartilages of the sharks. The premaxillaries sometimes coalesce into a single piece (as in Diodon, Mormyrus), or they are firmly united with the maxillaries (as in all Gymnodonts, Serrasalmo, &c.). The relative position and connexion of these two bones differ much, and form a valuable character in the discrimination of the various families. In some, the front margin of the jaw is formed by the premaxillary only, the two bones having a parallel position, as in the perch, in which case the maxillary is constantly toothless ; in others, the premaxillary is shortened, allowing the maxillary to enter, and to complete, the margin of the upper jaw ; and finally, in many, no part of the maxillary is situated behind the premaxillary, but the entire bone is attached to the end of the premaxillary, forming its continuation. In the last case the maxillary may be quite abortive. The mobility of the upper jaw is greatest in those fishes in which the premaxillary alone forms its margin. The form of the premaxillary is subject to great variation : the beak of Belone and Xipliias is formed by the prolonged and coalesced premaxillaries. The maxillary consists sometimes of one piece, sometimes of two or three. The principal membrane-bone of the mandible is the dentary (34), to which is added the angular (36) and rarely a smaller one, the splenial or os opercularc, which is situated at the inner side of the articulary. 5. Cartilage-Bones of the Respiratory Portion of the V isceral Skeleton of the Skull. With few exceptions all the ossifications of the hyoid and branchial arches belong to this group. The hyoid arch is suspended by a slender styliform bone, the stylohyal (29), from the hyomandibulars ; it consists of three segments, the epihyal (37), the ceratohyal (38), which is the longest and strongest piece, and the basihyal, which is formed by two juxtaposed pieces (39, 40). Be tween the latter there is a median styliform ossicle (41), extending forwards into the substance of the tongue, called the glossohyal or os linguale. The branchial arches (figs. 26 and 28) are enclosed within the hyoid arch, with which they are closely con nected at the base. In the perch and in the majority of Teleosteans they are five in number, of which four bear gills, whilst the fifth (56) remains dwarfed, and is beset with teeth ; it is called the lower pharyngeal bone. The arches adhere by their lower extremities to a chain of ossicles (53, 54, 55), the basibranchials, and, curving as they ascend, nearly meet at the base of the cranium, to which they