SEA-FISHERIES LABORATORY. 175 



teriorly, the apex of the lower arm overlapping the sub- 

 operculum. Its anterior extremity is greatly strength- 

 ened by a median ridge of bone deeply cupped in front 

 and forming a strong ball and socket joint with a process 

 on the posterior margin of the lryomandibular. 



Sub -operculum (S.Op.). — Described in the Sole by J". 

 T. Cunningham as the " Inter-opercular." A leafy bone 

 thinner than the operculum, sending upwards and back- 

 wards a long process behind the bifid margin of the 

 operculum. Yentrally it overlaps a small portion of the 

 inter-operculum. The operculum and sub-operculum 

 support the posterior free margin of the opercular fold, 

 and the characteristic posterior process at the base of the 

 pectoral fin (see fig. 23) is formed by the upper extremity 

 of the sub-operculum and the upper limb of the oper- 

 culum. 



Inter-operculum (I. Op.). — The " Sub-opercular " of 

 Cunningham. A thin bone but stouter than the sub- 

 operculum. It stiffens the ventral free margin of the 

 opercular fold. The whole of its dorsal edge lies under 

 the pre-operculum, and at about the middle of this edge 

 there is a depression (and here, as in the operculum, the 

 bone is thickest and most strong), providing a ligamentous 

 articulation with the inter- and epi-hyals at the junction 

 of the two latter — a somewhat similar condition to that 

 found in Amia.f The connection of the operculum and 

 inter-operculum (and especially the latter) with the hyoid 

 arch, both apparently common in the bony fishes, confirms 

 the view that these elements are modified branchiostegal 

 rays. The sub-operculum is probably also another. 



Pre-operculum {P. Op.). — This is usually considered 

 to be a modified lateral line bone, i.e., a bone developed 

 primarily around a portion of the lateral line system, and 

 f Allis, Jour. Morph., xii. Cp. also Shufeldt, Report U.S. F. C, 1883. 



