The largest uppermost flat bone of the gill-covers is known as the _opercle_ (25). Below it, joined by a suture, is the _subopercle_ (26). Before it is the prominent ridge of the _preopercle_ (24), which curves forward below and forms a more or less distinct angle, often armed with serrations or spines. In some cases this armature is very highly developed. The _interopercle_ (27) lies below the preopercle and parallel with the lower limb. =Branchial Bones.=--The bones of the branchial apparatus or gills are very numerous and complex, as well as subject to important variations. In many fishes some of these bones are coossified, and in other cases some are wanting. The tongue may be considered as belonging to this series, as the bones of the gills are attached to its axis below. In the striped bass, as in most fishes, the tongue, gristly and immovable, is formed anteriorly by a bone called the _glossohyal_ (37). Behind this are the _basihyals_ (36), and still farther back, on the side, is the _ceratohyal_ (35). To the basihyals is attached a bone extending downward and free behind the _urohyal_ (38). Behind the ceratohyal and continuous with it is the _epihyal_ (34), to which behind is attached the narrow _interhyal_ (33). On the under surface of the _ceratohyal_ and the _epihyal_ are attached the _branchiostegals_ (39). These are slender rays supporting a membrane beneath the gills, seven in number on each side in the striped bass, but much more numerous in some groups of fishes. The gill membranes connecting the branchiostegals are in the striped bass entirely separate from each other. In other fishes they may be broadly joined across the fleshy interspace between the gill-openings, known as the _isthmus_, or again they may be grown fast to the isthmus itself, so that the gill-openings of the two sides are widely separated. =The Gill-arches.=--The gills are attached to four bony arches with a fifth of the same nature, but totally modified by the presence of teeth, and very rarely having on it any of the gill-fringes. The fifth arch thus modified to serve in mastication instead of respiration is known collectively as the _lower pharyngeals_ (46). Opposite these are the _upper pharyngeals_ (45).