COMPARATIVE STUDY OF SCOMBROID FISHES. 
325 
connecting the anterior and posterior angles are strengthened by thick 
ridges. In the Scombridæ the opercle is thin, rather narrow, and the lower 
angle is acute, while the upper and posterior angles are rounded. The dorsal 
posterior side lias an indentation just above the posterior angle. The dorsal 
portion i. e. the portion above the horizontal diagonal is smaller than the 
ventral portion. The articular cup is more or less rounded with a sharp tooth 
at the anterior dorsal margin. In the Cybiidæ the opercle is rather broad, 
and more or less pentagonal. The dorsal portion is smaller and thinner than 
the ventral. The dorsal angle is rounded, and the posterior sides are more 
or less serrated. The ventral anterior side is not straight. The articular cup 
is narrow and elongate. In the Plecostei the opercle is thin but firm, and 
nearly quadrate in form, so that the dorsal and ventral portions are nearly 
equal to each other. The dorsal anterior side is concave. The articular cup 
is ellipsoidal. 
The subopercle is more or less triangular, its upper side being overlapped 
by the opercle, and the anterior side by the interopercle, while the posterior 
side remains free. In the Scombridæ the subopercle is very narrow, and 
bifurcated at the dorsal end. The anterior branch is short and pointed. In 
the Cybiidæ the bone is broad, its anterior branch is also broad and some¬ 
times two-horned, except in .Sarda and Gymnosarda. In the Thunnidæ the 
anterior branch is abortive and the whole bone is nearly obovate. In Thunnus 
orientalis and also in Th. thymus of the Altantic, the subopercle is more or 
less crenulated or concave at the anterior margin ; but in other tunnies the 
anterior margin of the subopercle is convex. In the Katsuwonidæ the anterior 
branch is produced anteriorly and nearly horizontally, ending with a blunt 
end. 
The interopercle is an ovate bone, forming the ventral free margin of the 
gill-cover with fine serrature. The bone is connected by a ligament to the 
posterior end of the hyoid arch. The interopercle of Thunnus orientalis has 
its posterior margin convex, while that of the other Japanese tunnies has its 
posterior margin nearly straight. 
The preopercle is a large bent bone, of which the vertical limb fits 
closely against a groove of the outer margin of the hyomandibular, and the 
horizontal limb to the metapterygoid and quadrate. In the Scombridæ this 
