520 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 
ANALYTICAL KBY TO THE SUBGENERA AND SPECIES OF HYMBENOCEPHALUS.? 
A*. 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*. 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. 
HY MENOGADUS. 
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’, 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=2222— = tenuis. 
B®. 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’. Barbel much longer than the orbit; ventral fins with 8 rays. 
b*. Pectoral rays 11; barbel two-thirds length of head_______ longibarbis. 
b?. Pectoral rays 15; barbel less than two-thirds length of head; snout 
and maxillary longer, and eye smaller than in Giinther’s figure of 
LON QUO OT O18 a ee ae nee ve ed Cl longiceps. 
a. Barbel much shorter than orbit, often obsolete. 
c’. Ventral rays greatly produced beyond anus, 8 in number; no 
barbel. 22 he oe ee ee ee longipes. 
ce. Ventral rays little or not at all produced beyond anus. 
ad’. Ventral rays 7 or 8 (rarely 6 or 9); snout scarcely produced be- 
yond the mouth. 
e*’. Barbel developed; interorbital width about equal to length of 
OLbith Or 6SSei2. 2 eae ah ees 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 
lene thioThnead 22 = 2 ae ee Seat s. striatissimus. 
g°. 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 
DUpI] 22a Boy ee Se ae ee ee eee s. torvus. 
e’. Barbel absent; interorbital space one-third wider than eye; ven- 
EDA) yA VST See noes ee eI ae ee grimaldii. 
@. Ventral rays 10 to 14. 
h*. Orbit 2.5 to 3.25 in head; its length greater than the interorbital 
width. : 
#. Snout much shorter than the interorbital width, not projecting 
beyond the mouth, rounded. 
7. 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 
H. longibarbis and H. grimaldii. 
