NORTH SEA INVESTIGATIONS. 185 
fishermen, with what truth I know not; and in naming the species 
R. blanda we have made use of the Latin epithet which assimilates 
most closely in sound to the vernacular name in general use. 
In conclusion we may be permitted to express our indebtedness 
to Dr. Giinther for advice and references, and especially for invalu- 
able aid in the comparison of specimens at the British Museum. 
Indeed, I may add that it was chiefly at his instigation that those 
inquiries were instituted which have now resulted, as I trust, in 
finally establishing the distinctness of the two species. 
IV. Tue “ Recessus Orpiraris,’’? an Accessory VisuAL OrGAN IN 
Pievronectip FIsHEs. 
I have applied the above name toa structure described in a paper 
read before the Zoological Society, of which the present remarks are 
a brief abstract. It first came under my notice when examining the 
cephalic anatomy of a common sole, but as I could find no mention 
of it in Cunningham’s Treatise on that species, | supposed that 
the specimen before me might be exceptional. It proved, however, 
‘ to occur regularly, not only in the sole, but in all other Pleuro- 
nectids which I examined, viz. the halibut, long rough dab, brill, 
plaice, flounder, and lemon “sole.” The material at my command 
proving unsuitable for a study of its development, I am unable to 
form any very valid opinion as to its homologies, and can only put 
forward what I know of its condition in the adult. 
The recessus in those species in which it is most conspicuously 
developed consists of a diverticulum of the membranous wall of the 
orbital cavity. It is sac-like in form, with very delicate membranous 
walls, except where the latter are traversed by anastomosing 
muscular bands. Such bands may also cross the lumen of the sac, 
and may even form more or less complete septa. The recessus of 
the lower eye lies below the skin immediately behind the eye, 
occupying a space bounded by the anterior face of the superficial 
jaw muscles, the backward continuation of the interorbital septum 
and the membranous wall of the orbital cavity. With this cavity it 
communicates by one or more rather large openings with thickened 
rims, and frequently by other smaller openings, the whole number 
being rather close together, and variable within the limits of a single 
species. In the brill the recessus of this eye is in the form of a 
conical process of the membranous wall, of which only the apex is 
strengthened by internal muscular apparatus. In the halibut there 
is no definite sac in connection with the lower orbit, but a portion 
of the membranous wall is differentiated by the development of 
numerous interlacing muscular bands, backed by very delicate mem- 
NEW SERIES.—VOL. III, NO. II. 15 
