dl 
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 hyomandibular. 
Sub-operculum (S.0.).—Described in the Sole by J. 
T. Cunningham as the “ Inter-opercular.” A jeafy bone 
thinner than the operculum, sending upwards and back- 
wards a long process behind the bifid margin of the 
operculum. Ventrally 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-opereulum and the upper limb of the oper- 
culum. 
Inter-operculum (/.0p.).—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 Hes 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.t 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?.0p.).—This is usually considered 
to be a modified lateral line bone, 7.e., a bone developed 
primarily around a portion of the lateral line system, and 
+ Allis, Jour. Morph., xii. Cp. also Shufeldt, Report U.S. F. C., 1883. 
