The _mandible_ or lower jaw consists of a right and left ramus; their union by a ligament in front is called _symphysis_. Each ramus is formed of several pieces; that which, by a sigmoid concavity articulates with the quadrate, is the _articulary_ bone (35); it sends upwards a coronoid process, to which a ligament from the maxillary and the masticatory muscles are attached; and forwards a long-pointed process, to be sheathed in the deep notch of the anterior piece. A small separate piece (36) at the lower posterior angle of the mandible is termed _angular_. The largest piece (34) is tooth-bearing, and hence termed _dentary_; at its inner surface it is always deeply excavated, to receive a cylindrical cartilage, called _Meckel’s cartilage_, the remains of an embryonic condition of the jaw, the articulary and angular being but ossified parts of it. In other _Teleostei_ this number is still more increased by a _splenial_ and other bones. The _infraorbital_ ring of bones (Fig. 23, ^{19}) consists of several (four) pieces, of which the anterior is the largest, and distinguished as _præorbital_. The so-called _præoperculum_ (30) belongs rather to the bones of the suspensorium of the mandible, presently to be described, than to the opercles proper. It is narrow, strong, angularly bent, so as to consist of a vertical and horizontal limb, with an incompletely closed canal running along both limbs. As it is quite a superficial bone, and frequently armed with various spines, its form and configuration form an important item in the descriptive details of many fishes. The principal piece of the gill-cover is the _operculum_ (28), triangular in shape, situated behind, and movably united with, the vertical limb of the præoperculum. There is an articulary cavity at its upper anterior angle for its junction with the hyomandibular. The oblong lamella below the operculum is the _sub-operculum_ (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.