gilbert: the lantern -fishes of japan 81 



10. Myctophum pterotum (Alcock). 

 Scopelus {Myctophum) pterotus Alcock, Ann. Mag. Nat. Hist., 1890, p. 217; 



Indian Ocean. 

 Myctophum gilberti, Evermann and Seale, Bull. U. S. Bur. Fish., 1907, p. 55; 



Philippine Islands. 



Sixty-five specimens were collected by Professor J. O. Snyder in the 

 market at Kagoshima (C. M. No. 4600), and have been examined with reference 

 to the excessive variation which Brauer has reported in this species. No indi- 

 cation of such variation is given in the Japanese material, nor in numerous 

 specimens from the Eastern Pacific, with which we have compared it. On the 

 contrary, both as regards number and position of the photophores, this seems 

 to be a very constant type. Brauer's results were partly based on his failure 

 to distinguish between M. pterotum and M. fibulatum Gilbert and Cramer, the 

 latter from the Hawaiian Islands. The two species are perfectly distinct and 

 can be recognized at a glance at any age after the photophores have developed. 

 Brauer's text-figure 93 (Die Tiefsee Fische, 1906, p. 182) represents M. fibu- 

 latum and not M. pterotum, and may have been drawn from one of the specimens 

 which Brauer examined from the Hawaiian Islands. Much is lost in this 

 volume through the failure to specify the locality of specimens from which the 

 drawings were made, and in general to discuss what are conceived to be vari- 

 ations of the different forms in relation to their geographic distribution. 



M. pterotum is a diminutive species, the majority of mature specimens 

 ranging from 50 to 60 mm. in total length, none yet reported in excess of 70 

 mm. Mature females of 50 mm. are in the Japanese material. In neither males 

 nor females are there well-developed luminous plates on the caudal peduncle, 

 such as figured by Brauer (l. c, text-figure 94), but occasionally a very faint 

 single luminous scale can be detected on the back of the caudal peduncle in 

 both sexes. No luminous scales on the under surface of the caudal peduncle 

 are present in any specimens. 



In the arrangement of the photophores, M. pterotum differs constantly from 

 M. fibulatum in the following respects: 1. The first and second supra-anals, the 

 supraventral and the suprapectoral are in a straight line which passes obliquely 

 forwards and upwards from the second supra-anal. In M. fibulatum, the line of 

 the two lower supra-anals passes forwards and downwards, traversing the second 

 (elevated) ventral, and leaving both supraventral and suprapectoral far above it. 



2. Supraventral lower, its distance from lateral line half its distance from 

 base of ventrals (one- third its distance from base of ventrals in M. fibulatum). 



