Collette and Gillis: Osteological differences between two species of Orammatorcynus 



33 



a 



Figure 21 



Left opercles in lateral view, (a) Scomberomorns semifasciatus, New Guinea, 510mm FL; (b) Acan- 

 thocybium solandri, Revillagigecios Is., 1080mm FL; (c) Grammatorcynus bilirwatiis, Marshall Is., 424mm 

 FL; (d) G. bicarinatiis, Great Barrier Reef, 563 mm FL. 



Preopercle The preopercle is a large, crescent- 

 shaped flat bone, broadest at the lower posterior angle 

 (Collette and Russo 1985b: fig. 33). The anterior por- 

 tion of the bone is thickened into a bony ridge. A series 

 of 5-7 pores along the lower margin of the ridge 

 represents the preoperculomandibular canal of the 

 lateral line system which continues onto the dentary. 

 On the mesial side, the bony ridge possesses a groove 

 for attachment to the hyomandibula and the quadrate. 

 There is a shelf mesial to the anteroventral end of the 

 preopercle in Acanthocybium that is absent in Gram- 

 matorcynus and Scomheromorus. The canals leading 

 to the preopercular pores are visible through the bone 

 in G. hilineatus and all species oi Scomheromorus, but 

 these canals could not be seen in the specimens of 

 G. bicarinatus and Acanthocybium due to the thickness 



of the bone. The posterior margin of the preopercle is 

 distinctly concave in Grammatorcynus and most 

 species of Scomberomorus. However, it is only slight- 

 ly concave or flat in Acanthocybium and S. commer- 

 son. In Grammatorcynus the distance from the anterior 

 margin of the bony ridge to the posterior end of the 

 lower lobe is 64-75% of the height of the preopercle 

 measured from the ventral margin to the dorsal tip of 

 the bone. In Scomberomorus the lower lobe is 69-80% 

 of the height of the preopercle. The anterodorsal mar- 

 gin terminates in a pore similar to the preoperculo- 

 mandibular lateral-line canal pore at the anteroventral 

 margin of the bone. 



Subopercle The subopercle is a flat, roughly 

 triangular bone with a prominent anterior projec- 

 tion (Collette and Russo 1985b: fig. 34). Two ridges 



