Some authors have placed the species off Japan, but, as discussed below, an undescribed form 



may occupy those waters. 



Discussion 



A probably new, closely related species occurs in the northwestern Pacific from Japan to at 

 least 170° E. It differs from S. californiensis in having generally higher counts, with little or no 

 overlap, and a shorter head and upper jaw. Due to inadequate collections from the north- 

 central Pacific, it is not now possible to establish either a distinct break in distribution of the 

 two forms or a cline in the respective characters. 



Myctophum 



Rafinesque, 1810 



SAO series in a straight or slightly curved oblique line. SAOi well below level of SAO2 and 

 always behind a vertical from VO3; 1 Pol; 2 Pre. 



Key to species of Myctophum 



la. Pol behind a vertical from origin of base of adipose fin 2 



lb. Pol on or before a vertical from origin of base of adipose fin 4 



2a. Body short, deep, the greatest depth about 3 in SL. Least depth of caudal peduncle 2 or less 

 in the length. Body scales markedly ctenoid. SAO in a straight or slightly curved line. 



SAOi behind a vertical fromcenter of VO4 M. selenoides 



2b. Greatest body depth 4 or more in SL. Least depth of caudal peduncle much more than 2 in 



the length. Body scales with smooth or weakly crenulated margins 3 



3a. Posterodorsal margin of operculum markedly angulate, often recurved or hooked, very 

 slightly or not at all striate or dentate. All body scales with smooth margins. SAO in a 



straight line that passes behind VO.t M. nitidulum 



3b. Posterodorsal margin of operculum rounded; scales on anterolateral part of body weakly 

 crenulate, all others with smooth margins. SAO curved. A line through SAO1.2 passes 



through or before VO3 M. obtusirostrum 



4a. Upper Pre near lateral line, widely distant from lower Pre. Dorsolateral portion of opercle 

 moderately enlarged, the margin notably striate and dentate. SAO series in a straight line 



that passes over VO3. Pol far before origin of base of adipose fin M. phengodes 



4b. Upper Pre far below lateral line, near lower Pre 5 



5a. Body scales with smooth margins. Body slender, its greatest depth about 5.5 in SL. 

 Posterodorsal margin of operculum notably striate and dentate, particularly in large 



specimens , M. aurolaternatum 



5b. Body scales markedly ctenoid 6 



6a. Scales over AO photophores with 1 to 3 elongate, sharp spines that project posteriorly 7 



6b. Scales over AO photophores without elongate spines 8 



7a. SAO series straight, or aberrantly and very slightly angulate, a line through the series 

 passing through or very near VO3. Greatest depth of body 4.0 or less in SL. Least depth of 



caudal peduncle about 3.2 (3.1-3.4) in the length M. lychnobium 



7b. SAO series always slightly angulate. A line through SAO,.j passes before VO3, often 

 through VO2. Greatest body depth 4.0 to 4.5 in SL. Least depth of caudal peduncle about 



2.9 (2.8-3.0) in the length M.spinosum 



8a. Total gill rakers 14-17. A line through SAO, .2 passes variously between VO, and VO2, or 



before VO, M.asperum 



8b. Total gill rakers 27-29. A line through SAO1.2 passes variously through or between VO3 

 and VO2 M. brachygnathum 



53 



