178 THE VOYAGE OF H.M.S. CHALLENGER. 



walls appear thick and the mucous membrane laid into deep folds. Pyloric appendages 

 present, short, and in small number. Air-bladder long, thick-walled dorsally, but 

 thin and elastic along its ventral aspect. Towards the posterior extremity of the 

 bladder, which is subcylindrical and pointed, its wall is very thick, and of an almost 

 cartilaginous consistency behind ; its end is open, the circular aperture being thickened 

 and closed by what appears as a conical muscular mass, which, after it has penetrated 

 into the interior of the bladder, spreads out and forms a layer on the inside of its 

 posterior portion. An examination of fresh specimens will show the real condition of 

 this singular apparatus, the function of which is most likely connected with the com- 

 pression or expansion of the gas contained in the bladder. 



Photichthys argenteus (PI. XLV. fig. A). 



Phosichthys argenteus, Hutton, loc. cit., p. 56; and vol. v., pi. xv. fig. 90. 



B. 21. D. 13. A. 26. P. 9. V. 7. L. lat. 50? 



The height of the body is contained six and a half times in the total length, without 

 caudal, the length of the head four and a half times. The vent is nearly twice as distant 

 from the eye as from the root of the caudal. Eye of moderate size, one-fifth of the length 

 of the head, a little shorter than the snout, but as wide as the interorbital space. Cleft 

 of the mouth exceedingly wide, the maxillary extending to the posterior angle of the 

 pra3operculum ; lower jaw somewhat projecting beyond the upper. There is only one 

 infraorbital bone, but it is succeeded behind by two long and very thin supplementary 

 bones, which are attached along the upper margin of the maxillary. Cheek uncovered 

 either by scales or by bone. 



The branchiostegals are very short, shorter than the eye. Four branchial arches, 

 which bear extremely short gill-laminse, much shorter than the opposite gill-rakers. On 

 the outer branchial arch there are eleven of these slender, needle-shaped, and distantly 

 placed gill-rakers in its lower portion, and five in its upper. Pseudobranchise absent. 



The origin of the dorsal fin is somewhat nearer to the root of the caudal than to the 

 extremity of the snout, and the fin is nearer to the ventrals than to the anal. The 

 adipose fin is a very small and narrow lobe, which may be easily overlooked, or it may be 

 entirely absent. Anal fin much higher in front than behind, and terminating at some 

 distance from the caudal, the peduncle being long and deep. Caudal fin forked. 

 Pectorals narrow. Ventrals well developed, their root being equidistant from the vent 

 and the base of the pectorals ; they terminate at a long distance from the vent. 



I am unable to state anything about the scales, as only the traces of some of the 

 scale-pouches on the tail have been preserved. The isthmus is deep and much com- 

 pressed, and the scales on each side are so arranged that one corresponds to and covers 

 each of the luminous organs of this region. 



