57 
III. Drastaromma Burm. 
Ophiogomphus Selys. 
IV. HerprrocompPuus Selys. 
Diastatomma and Herpetogomphus are very closely related, 
and no good character has yet been found to separate them 
when in the nymphal stage. They are then similar in form to 
the common gomphid larve of the genera which follow, but 
may be easily distinguished from them all by the widely diverg- 
ing wing-pads, the inner margins of which separate at an angle 
of 60° or more instead of being approximately parallel as in 
the genera which follow. In exuvie of the latter forms, how- 
ever, the wing-pads are often more or less separated. Her- 
petogomphus has not been found east of the Mississippi. Déas- 
tatomma is widely distributed in North America, but both the 
nymphs and imagos are quite rare in I]linois, owing prob- 
ably to the small extent of rocky stream-beds and rapid cur- 
rents. The rivers near Rock Island and Goleonda, in which 
examples have been found, afford favorable situations of this 
kind. Three species of the nymphs are in our collections,—one 
found in Illinois,—readily separable by the number of lateral 
spines, the length of the lateral appendages, etc., as stated in 
the key. 
Nymph stout, little flattened. Head abruptly sloping for- 
ward from the ocelli, Labrum pilot-shaped. Antenne with 
the two basal segments globular, third segment twice as long 
as both basal, much flattened and laid close beside the labrum. 
Fourth joint a minute rudiment. Median lobe of mentum 
rounded, with border of short blunt teeth and a double series 
of fringing scales. Lateral lobes nearly straight, not terminat- 
ing in an end hook and minutely denticulated within ; movable 
hook short, arcuate. 
Legs rather short. Fore and middle tibiz with external 
hooks, wing-cases divaricate, strongly sloping downward toward 
the sides. Dorsal hooks on abdominal segments on 2 or 3 to 9. 
Tenth segment not inclosed by the 9th but triquetral, exceeding 
