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frontal and prefrontal above, and by the ethmoid cartilage 
below. As seen in the dried skull (fig. 3) these structures 
bound a large fenestra. A membranous sheet stretches 
across this fenestra from the ventral edge of the right 
frontal to the ethmoid cartilage and completes the septum. 
In front the ethmoid cartilage is perforated by a wide 
opening. This is the ethmoidal fenestra. It is seen in 
fig. 3, and through it the internal surface of the left pre- 
frontal may be seen in the figure. When the cranium is 
held dorsal surface uppermost the interorbital septum is 
almost exactly horizontal. 
The Recessus orbitalis.—This is the term applied by 
Holt* to an accessory of the organ of vision present in all 
Pleuronectid fishes.. It is an evagination of the mem- 
branous wall of each orbit forming a sac which lies outside 
the orbit and the cavity of which communicates with that 
of the former by one or more openings. The recessus of 
the right eye lies immediately behind the bulb and just 
underneath the interorbital septum. To expose it the 
skin must be very carefully removed from the region 
immediately behind the eye from the interorbital ridge 
downwards. On removing a little connective tissue the 
organ is then seen. It is a conical sac of fatty appearance 
with the apex directed backwards. In a fish of about 
22 inches in length it is about lem. in total length in the 
contracted condition. On cutting open its outer wall its 
cavity is seen to be somewhat reduced by bands of muscle 
fibres which cross it and are massed together on the 
internal wall. A seeker can be passed from its cavity into 
that of the orbit. On cutting away the skin round the 
eye and carefully removing the latter after dividing the 
optic nerve and eye muscles, the opening of the recessus 
can be séen immediately above the place where the 
*Holt—Proc. Zool. Soc., No. 29, 1894, pp. 413-446 
