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



The priucipal distinctive feature of tliis species is that it possesses not only the long 

 detached upper pectoral ray, but also an equally long lower ray, which, however, is not 

 removed from the remainder of the fin. The upper ray becomes bifid in its proximate 

 third, and is accompanied by an extremely short rudimentary second ray ; it scarcely 

 reaches to the caudal fin and is shorter than the lowermost ray, ^^•hich may reach to the 

 end of the caudal and is split only at its very extremity. 



The structure of the ventral fins is the same as in Bathypterois longipes, but the 

 strong outer rays are somewhat curved and do not reach the end of the anal. 



The scales are cycloid as in the other species, but those behind the basal portion of 

 the pectoral fin are deeply pectinated, provided with from five to ten long and narrow 

 teeth, of which the middle ones are the longest (see figures). 



Colour blackish or black ; pectoral filaments whitish. 



Ipnops. 



Ipnojis, Giintli., Ann. and Mag. Xat. Hist., 1878, vol. ii. p. 187. 



Body elongate, subcylindrical, covered with large thin deciduous scales, and without 

 luminous organs. Head depressed, with broad, long, spatulate snout, the whole upper 

 surface of which is occupied by a pair of large transparent lamelliform membrane 

 bones which cover a luminous organ longitudinally divided into two symmetrical halves. 

 Eyes absent ; nostril reduced to a minute opening in front of each lamina. Bones of 

 the lower and lateral parts of the head well ossified. Mouth wide, with the lower jaw 

 projecting ; maxillary dilated behind. Both jaws with narrow bands of villiform teeth ; 

 palate toothless. Pectoral and ventral fins well developed, and, owing to the shortness 

 of the trunk, close together. Dorsal fin at a short distance behind the vent ; adipose fin 

 absent ; anal fin moderately long ; caudal subtruncated. Pseudobranchise absent ; air- 

 bladder absent ; pyloric appendages absent. 



This genus is closely allied to Bathypterois, not only in its external characters, but 

 also in the structure of its abdominal organs. Professor Moseley has kindly acceded to 

 my request to make a histological examination of the remarkable organ on the upper 

 surface of the head. The results of his examination are given in Appendix A and 

 Pis. LXVIL, LXVIII. 



Ever since the discovery of this fish much uncertainty has prevailed with regard to 

 the nature and function of the extraordinary apparatus on the upper side of the head ; 

 but from Professor Moseley's examination it seems to be almo.st beyond doubt, that it is a 

 special form of phosphorescent organ. The power of producing light, and thereby 

 attracting other creatures, must be of great use to a fish, which, deprived of organs of 

 sight and touch, would be unable to procure its food. 



