of the Fislieri) Boaulfor Scotland. 269 



The suboperculum is overlapped by the operculum, preoperculum, and 

 interoperculum. The bones are bound together by ligaments. In the 

 cod there is a distinct articulation between the hind end of the inter- 

 operculum and the anterior edge of the suboperculum, whereas in the 

 saithe and lythe the posterior extremity of the former simply overlaps 

 the front end of the former. 



Interoperculum (in. Op.). 



Cod, Fig. 30, PL X. : Saithe, Fig. 4, PI. X. : Lythe, Fig. 20, PI. IV. 



The interoperculum is a long and rather narrow bone. In the cod it is 

 almost completely ossified, only the hind inferior area remaining mem- 

 branous, while in the lythe and saithe a larger portion — almost the half 

 in the case of the latter — is membranous. At their narrow anterior 

 extremities, where the strong ligament binding the interoperculum to the 

 angular is attached, the bones of the three species are ossified. In the cod 

 the interoperculum is broadest at the middle of its length, and narrows to 

 either end. In the lythe the middle portion and the posterior end are of 

 about equal breadth, whereas in the saithe the hind extremity is rather 

 broader than the middle part. 



On the inner surface of the interoperculum there is a socket which 

 receives the head of the epihyal at its articulation with the inferior end 

 of the stylohyal. In the edge of the bone, just above the articular 

 socket, there is a notch or depressed portion very well marked in the 

 saithe, not so distinct in the lythe, and in the cod practically obliterated. 



The saithe and lythe differ in respect to the form of the articular 

 socket on the inner surface of the bone. In both forms the posterior 

 border or rim of the socket is prominent, but while in the saithe it 

 stands almost perpendicularly to the plane of the bone, in the lythe it 

 rises at a very small angle. In the cod this rim rises at a greater angle 

 than in the lythe, in some specimens of cod nearly approaching a right 

 angle. In the cod there is behind the socket a large rounded 

 prominence. 



The interoperculum articulates in front by means of a ligament with 

 the angular ; it is overlapped superiorly by the preoperculum ; it over- 

 laps the suboperculum, and it articulates by its internal surface with the 

 epihyal. 



Branchiostegal Arch. 



Basihyal (b. Hy.) : Ceratohyal (cer. Hy.) : Epihrjal (e[). Ily.) : 

 Stylohyal (st. Hy.). 



Cod, Figs. 45-48, PI. X. : Saithe, Figs. 20-23, PI. X. : 

 Lythe, Fig. 14, PI. IV. 



The basihyals differ a little in the three species. The larger, viz. the 

 ventral one, is in the lythe, in side view, seen to be roughly of a 

 parallelogram shape, whereas in the saithe it is triangular. The form in 

 the lythe is due to the hind end being extended backwards over the 

 anterior end of the ceratohyal in a triangular expansion. The head of 

 this basihyal, which is bound by ligament to its fellow of the opposite 

 side, is in the cod marked off by a neck-like constriction, which is not 

 present in the two other species. 



The ceratohyals do not show great differences. In general .shape that 



