520 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



ANALYTICAL KEY TO THE SUBGENERA AND SPECIES OP HYMENOCEPHALUS. 1 



A 1 . Body and head slender or moderately robust ; sensory canal system of 



head not excessively developed ; bony crests of skull thin, but firmer ; 



eye very large, being contained from 2.5 to 3.5 times in length of head ; 



color chiefly silvery. 



B 1 . Second dorsal spine weakly denticulate; body more slender; the head 



not compressed, its width equal to greatest depth of body or of head ; 



gill-rakers tubercular, about 10 to 13 on the lower limb of the outer 



two arches ; scales wholly smooth, so far as known. 



HYMENOGADUS. 



a\ Lens-shaped organ before anus circular in outline; striations on sides 

 of isthmus well developed ; gill-rakers about 12 in number on lower 



limb of outer two arches ; barbel nearly as long as orbit gracilis. 



a 2 . Lens-shaped organ before anus elongate; striations on sides of isthmus 

 obsolescent ; gill-rakers about 10 in number on lower limb of outer 



arches; barbel not quite half length of orbit tenuis. 



B a . Second dorsal spine wholly smooth ; body more robust ; head compressed, 

 its width being less than depth of body or of head ; gill-rakers short 

 but not tubercular, about 18 in number on outer arches ; scales with 



short spinules arranged in quincunx order hymenocephalus. 



a 1 . Barbel much longer than the orbit ; ventral fins with 8 rays. 



b 1 . Pectoral rays 11; barbel two-thirds length of head longibarbis. 



b 2 . Pectoral rays 15 ; barbel less than two-thirds length of head ; snout 

 and maxillary longer, and eye smaller than in Giinther's figure of 



longibarbis longiceps. 



a 2 . Barbel much shorter than orbit, often obsolete. 



&. Ventral rays greatly produced beyond anus, 8 in number ; no 



barbel longipes. 



c 2 . Ventral rays little or not at all produced beyond anus. 



d 1 . Ventral rays 7 or 8 (rarely 6 or 9) ; snout scarcely produced be- 

 yond the mouth. 

 e 1 . Barbel developed ; interorbital width about equal to length of 



orbit, or less striatissimus. 



f. Ventral rays 8 (rarely 7 or 9) ; barbel usually about as long 

 as pupil, or longer. 

 g\ Orbit nearly circular, contained 0.9 to 1.1 times in postorbital 



length of head s. striatissimus. 



g 2 . Orbit obliquely oval, its length contained 1.15 to 1.4 times in 



postorbital length of head s. aeger. 



f. Ventral rays 7 (rarely 6, never 8) ; barbel usually shorter than 



pupil s. torvus. 



e 2 . Barbel absent ; interorbital space one-third wider than eye ; ven- 

 tral rays 8 grimaldii. 



d 2 . Ventral rays 10 to 14. 



h 1 . Orbit 2.5 to 3.25 in head ; its length greater than the interorbital 

 width. 

 i 1 . Snout much shorter than the interorbital width, not projecting 

 beyond the mouth, rounded. 

 j 1 . Barbel small, but distinctly developed ; a small, distinct, 

 faintly striated area directly below base of pectorals, and 

 one before base of each ventral. 



1 The authors have examined all of the 19 species and subspecies now known, excepting 

 //. longibarbis and //. grimaldii. 



