58 
the lateral spines, its own lateral margin forming a part of the 
margin of the abdomen. 
KEY TO THE NYMPHS OF DIASTATOMMA HEREIN DESCRIBED. 
Lateral spines on abdominal segments 7-9. 
Dorsal hooks distinct, narrowly prominent, ending ina pointed 
tooth ; abdominal granulations fine and much less con- 
spicuous than in the next species; male superior append- 
age scarcely tubercled, lateral appendages three fourths 
to four fifths as long. 1. severus. 
Dorsal hooks represented by a broad elevation, ending poste- 
riorly on each segment in a rounded tooth; abdomen 
evenly dotted with rather coarse blackish granulations; 
superior appendage of male notched back of middle, in 
front of which is a pair of tubercles, laterals about three 
fifths as long as the superior one. 3. carolus. 
Lateral spines on abdominal segments 6-9; dorsal hooks as in 
carolus but much more prominent; male superior append- 
age scarcely tubercled, laterals nine tenths as long as 
the superior. 4. sp. (a). 
1. Diastatomma severus Hagen. 
Ophiogomphus severus Hagen. 
The nymphs here described were collected by Dr. Forbes 
in large numbers from streams in Yellowstone Park, such as 
the Fire Hole River, Nez Perce Creek, and Gibbon and Good- 
win rivers, during the latter half of August. They were found 
on sand under stones in shallow rapids, and on weedy and 
grassy bottoms. They answer well to the description given by 
Hagen (’85, p. 259) for severus, which is by far the most abun- 
dant gomphid in that region. 
The nymph, apparently grown, measures 25 mm.; the 
abdomen, 16 mm.; hind femur, 5 mm.; width of abdomen 7mm., 
of head 5 mm. 
Color (alcoholic specimens) fulvous yellowish beneath and 
on the sutures, and on the legs beyond the middle of the fem- 
