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Fishery Bulletin 96(3), 1 998 
slightly because the posterior region of the red hake 
opercle is longer in relation to the body of the opercle 
than that of silver hake and is also narrower, ending 
in a sharper point in contrast with the opercle of sil- 
ver hake. The opercles of Atlantic cod ( Gadus 
morhua ) and haddock (. Melanogrammus aeglefinus) 
share a general triangular shape with the hakes, 
although it is less pronounced in cod and haddock 
(Fig. 1, C and D). Opercles of Atlantic cod and had- 
dock can be further separated from hake opercles by 
the presence of only one prominent ridge, which ex- 
tends posteriorly from the site of hyomandibular at- 
tachment, rather than the two ridges found in hake. 
There appear to be no consistent, observable differ- 
ences between the opercles of Atlantic cod and had- 
dock. The opercles of alewife ( Alosa pseudoharengus) 
and Atlantic herring ( Clupea harengus ) are not tri- 
angular and have a distinct notch that is dorsal to 
the site of hyomandibular articulation along the dor- 
sal margin, with a pronounced curvature along the 
posterior margin (Fig. 1, E and F). The opercles of 
alewife and Atlantic herring each possess one ridge, 
extending ventrally from the site of hyomandibular 
articulation along the anterior margin, and each also 
has four definable margins. Alewife and Atlantic 
herring opercles differ mainly in the shape of the 
dorsal notch — the notch being cup-shaped in alewife, 
compared with v-shaped in Atlantic herring. In At- 
lantic mackerel (Scomber scombrus), the opercle has 
a pronounced dorsal curvature with a notch located 
centrally along the posterior margin (Fig. 1G). The 
ridge extending from the hyomandibular articulation 
site is unique to Atlantic mackerel in that it branches 
out broadly as it extends posteriorly from the articu- 
lation site. The opercles of butterfish ( Peprilus 
triacanthus) are unique in general shape, resembling 
a butterfly wing, with four clearly defined margins 
and a ridge extending ventrally from the hyomandib- 
ular articulation site along the anterior margin (Fig. 
1H). Sand lance (Ammodytes dubius ) represent the 
only species examined here with opercles not possess- 
ing ridges along any margins. Sand lance opercles also 
