Page:An Introduction to the Study of Fishes.djvu/79

Rh articulary cavity at its upper anterior angle for its junction with the hyomandibular. The oblong lamella below the operculum is the suboperculum (32), and the one in front of this latter, below the horizontal limb of the præoperculum, is the interoperculum (33), which is connected by ligament with the angular piece of the lower jaw, and is also attached to the outer face of the hyoid, so that the gill-covers cannot open or shut without the hyoid apparatus executing a corresponding movement.

The chain of flat bones which, after the removal of the temporal muscles, appear arranged within the inner concavity of the præoperculum (Fig. 24), are comprised with the latter under the common name of mandibulary suspensorium. They connect the mandible with the cranium. 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; is entirely cartilaginous in the young, but nearly entirely ossified in the adult. The position of this bone is noteworthy, because, directly inwards of its cartilaginous junction with the hyomandibular, there is situated the uppermost piece of the hyoid arch, the stylo-hyal. 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 teleosteous sub-class. Finally, the large triangular hypotympanic or quadrate (26) has a large condyle for the mandibulary joint.

The palatine arch (Fig. 26) connects the suspensorium with the anterior extremity of the skull, and is formed by three bones: the entopterygoid (25), 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