320 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



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 1cm. 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 seen immediately above the place where the 

 ♦Holt— Proc. Zool. Soc, No. 29, 1894, pp. 413-446 



