268 Part III. — Twentieth Annual Report 



The preoperculum articulates anteriorly and superiorly with the front 

 and median processes of the hyomandibnlar, and with the syniplectic and 

 quadrate inferiorly ; behind it is bound by ligaments to the operculum, 

 suboperculum, and interoperculum. On the internal surface of the 

 median anterior expansion there is a hollow which receives the head of 

 the stylohyal. This cavity is bounded by the ends of the hyomandibular 

 and symplectic processes. 



Operculum. (Op.). 



Cou, Fig. 33, PI. X. ; Saithe, Fig. 7, PI. X. : Lythe, Fig. 27, PI. IV. 



The shape of this bone differs in the three species. It is cleft on its 

 posterior border by a more or less deep angle. In the cod the angle is 

 deep, and the two lobes thus formed are very different from one another. 

 The superior is triangular and ends in a narrow apex ; the lower is 

 broader and mitre-shaped. In the saithe and lythe the angle enters 

 slightly, in the saithe less than in the lythe. 



The socket for articulation with the hind process of the hyomandibular 

 is in the anterior upper corner. The superior edge of the bone runs 

 nearly parallel with the axis of the socket. The anterior and inferior 

 edges are in one curve ending in the apex of the broad lobe. 



In the lythe the two lobes of the operculum are of nearly the same 

 size, and except for the widely arched inferior edge it is somewhat diffi- 

 cult to distinguish the two. The bone of the saithe is readily separated 

 from the other two by the great development of the lower lobe. In large 

 specimens the lower lobe is nearly square. The anterior edge of the 

 operculum passes from the articular socket vertically downwards, and 

 then curves backwards to join the inferior edge. In the cod and saithe 

 there is in the anterior superior corner just above the socket a prominent 

 angle. In the lythe it is absent. The operculum is joined to the pre- 

 operculum and suboperculum by ligaments ; it overlaps the latter bone. 



Suboperculum (sb. Op.). 



Cod, Fig. 32, PL X. : Saithe, Fig. 6, PI X. : Lythe, Fig. 28, PI. IV. 



The suboperculum of the cod is separated from that bone in the saithe 

 and lythe by the fact that there is in its anterior edge a notch, in which 

 articulates the bind end of the interoperculum. The sul)opercula of 

 the saithe and lythe rather resemble one another. They are shoe-shaped, 

 but while in the case of the lythe the " instep " part is high, in the saithe 

 it is low. In the lythe the upper edge is gently, in the saithe more 

 deeply, curved downwards. The hollow in the upper edge is the part 

 where the lower lobe of the operculum overlaps the suboper(;uliim. In 

 the cod and lythe the suboperculum ends in a broadly rounded hind 

 extremity, while in the saithe the bone tapers away gradually to a much 

 narrower end than in the former two fishes. The extremity of the sub- 

 operculum is in the cod of about the same breadth as that in the lythe, 

 and in general shape the subopercula of the cod and lythe resemble one 

 another more than they do that of the saithe. The shape of the hind 

 portion of the suboperculum has a direct relation to the shape of tlie 

 oi)erculum. Thus the breadth of the hind end of the former depends on 

 the depth of the angle between the two lobes of the operculum. The 

 extremity of the suboperculum fills up the gap which the angle in the 

 operculum makes in the opercular plate. 



