GREGORY: FISH SKULLS 427 



canal, it seems at first that the course and position of the preopercular canal are the chief 

 factors in determining the location of the preopercular itself. Indeed Ridewood (1904a, p. 

 68), following Cole and Johnstone (1902, p. 175), classifies the preopercular with the 

 lacrymal, nasal, suborbital and supratemporal (pterotic, etc.) as bones "developed primarily 

 around a portion of the lateral-line system," and therefore of a different nature from the 

 other opercular bones. On the other hand, the preopercular fold seems to have essentially 

 the same relation to the quadrate and metapterygoid that the opercular fold (including the 

 inter- and sub-operculars as well as the opercular itself) has to the segments of the hyoid 

 arch; moreover, it is only in such advanced groups as the scorpsenoids that the preopercular 

 seems to be so closely dependent upon the tunnel of the latero-sensory canal that perforates 

 it. In the older ganoids the texture and general appearance of the preopercular is close to 

 those of the postorbital and opercular series, which do not carry latero-sensory canals. 

 Again, when the quadrate-articular joint is shifted forward, as in many small-mouthed fishes, 

 the lower end of the preopercular follows it; the upper end also follows the general line of 

 the suspensorium. Hence I conclude that the position of the suspensorium and the close 

 appression of the preopercular to the eye in the embryo are the leading factors in the position 

 of the preopercular and that the position and course of the preopercular-sensory canal are 

 secondary factors. Finally the exact form of the concave anterior border of the preopercular 

 is probably conditioned in part at least by the position of the three subdivisions (Al Al A3) 

 of the adductor mandibulse muscles, which are fastened to the border of the preopercular 

 and the outer surface of the metapterygoid (see page 425 above). 



From the posterior corner or elbow of the preopercular a single backwardly-directed 

 spike sometimes attains great size, as in Holocentrus (Fig. 112), Prionotus (Fig. 218), 

 Dactylopterus (Fig. 223), Callionymus (Fig. 242), Gobiesox (Fig. 249). Such spikes have 

 the appearance of being useful either as defensive weapons (when moved by the wriggling of 

 the body), or as "skids" upon which the body rests {Dactylopterus (Fig. 223)), or as acces- 

 sory braces for a pelvic adhesive organ {Gobiesox (Fig. 250), Callionymus (Fig. .242)). 

 Apparently such projecting spikes on the posterior border of the opercular have always 

 originated from one of the less prominent spikes of more primitive acanthopts. These 

 projections often start from near the center of growth of the preopercular and may have 

 arisen in the first place as strengthening ribs or projections on that part of the bone which 

 is subjected to the heaviest stresses (Figs. 114, 202). 



In the pediculates the opercular apparatus as a whole is naturally much influenced by 

 the great expansion of the pharynx (Figs. 265, 267, 272); here the opercular itself is reduced 

 to a thin scale supported by two long diverging tracts representing the upper and lower 

 spine of the opercular, while the subopercular, and still more the interopercular, are pro- 

 duced into trackers; the branchiostegals form very long, curved and delicate bands, which 

 follow the swelling skin flap that forms the chief functional element in respiration. 



In short, the bones of the opercular region teach us very clearly that they are only 

 local precipitates in growing flexible membrane and that their boundaries are predetermined 

 by the location of the movable creases in the membrane itself. At the same time it is 

 evident that the presence of each bone of the series is due to hereditary factors. Thus the 

 vast majority of fish inherit the normal elements of the opercular series, the only variability 

 being in the number of branchiostegal rays (see pages 135 and 231 above). 



Embryology of the Jaws, Hyoid Arch and Opercular Series. — W. K. Parker has figured the 



