426 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



The opercular bone is small In the primitive palseoniscids, which have a large sub- 

 opercular (Fig. \2A), while in the sturgeons the large rounded "opercular" has no contact 

 with the hyomandibular and appears to be an hypertrophied subopercular (see p. 119). 

 In the morays (Fig. 825) the upper two-thirds of the opercular series has been sacrificed in 

 order to give room for the swelling movements of the branchial syringe. In the gonostomids 

 (Fig. 53) the opercular system has become extremely narrow and in the gastrostomids it has 

 disappeared, probably in connection with the great distensibility of the mouth and throat. 

 In all other teleosts, so far as I know, the opercular retains its normal articulation with the 

 pedicle of the hyomandibular. The opercular region is enlarged in Ophiocephalus (Fig. 145), 

 Osphronemus (Fig. 147) and other fishes with enlarged respiratory chamber. 



The opercular flap often extends posteriorly beyond the opercular bone and the curve 

 of the posterior bevelled borders of the opercular and subopercular is always adjusted to 

 the curve of the cleithrum so that there is a smooth fit, which is also insured by the flexi- 

 bility of the border of the opercular flap and by the oblique position of the anterior surface 

 of the cleithrum. The two projecting processes on the posterior border of the opercular 

 (Frgs. 114, 201) separate the spiracular region above from the vertically movable bran- 

 chiostegal region below. The branchiostegal flaps act as pumps and valves for the rhythmic 

 escape of the inspired water and cooperate with the movements of the breathing valves, 

 which are folds of membrane on the fore part of the roof and floor of the mouth. 



It can readily be seen that the stresses set up by the dilatator, adductor and levator 

 operculi muscles (Fig. 285), together with the downward and forward pull of the interoper- 

 cular, might well tend to cause a buckling of the smooth contour of the opercular, with conse- 

 quent leaking of its valvular edge. This contingency is apparently eliminated in the typical 

 Acanthopterygii by the development of two divergent tracts of folded trabeculae (Fig. 202), 

 radiating from the fulcrum or operculo-hyomandibular contact, respectively to the farthest 

 points or projections on the upper posterior borders of the opercular. An oblique view of 

 this region in a living fish shows that during dilatation of the opercular the principal radiat- 

 ing ridge, which is continued posteriorly into the main opercular spine, is raised just ahead 

 of the flexible edge of skin beneath it. In the pediculate fishes (Figs. 267, 272, 279) these 

 two divergent tracts of bony trabeculae persist and stand out, even in very small specimens, 

 while the main part of the bone has become tenuous and translucent. 



The preopercular, although nominally belonging in the opercular series, lies between 

 the field of the adductor muscles of the jaw (Fig. 285) and the opercular flap and has func- 

 tional relations with these and other parts; it also supports the preopercular branch of the 

 latero-sensory canal (Fig. 203). It is usually a crescentic or boomerang-like bone which 

 follows the general curve of the suspensorium and fits between a vertical crest of the hyo- 

 mandibular and the anterior border of the opercular fold. In the embryonic and larval 

 stages (Fig. 204) the preopercular region is pressed close to the eye and doubtless its "cir- 

 cumorbital" position in the adult is partly determined by this early condition. At Its 

 lower end it fits behind the posterior crest on the quadrate; being concavo-convex as seen 

 from the outer side, it often seems to play an Important role In bracing and stiffening the 

 quadrato-articular joint. The posterior border is sometimes serrated (as in various per- 

 coids) or provided with a number of projecting spikes, which in scorpsenoids (Figs. 201, 212) 

 have the appearance of protecting the enlarged lateral line organs. 



From the almost invariable association of the preopercular with this latero-sensory 



