1863.] 
239 
genus, and that only observable in a few species, is the presence or non¬ 
presence of dorsal yellow vittse on the 9th and 10th abdominal joints, 
which is the more singular as the lateral yellow markings of joints 8 
and 9 are remarkably constant and afford one of the best specific cha¬ 
racters. We may add to this, as variations of a less marked character 
which also occur in certain species—1st. The confluence or non-conflu¬ 
ence, for a short space, near the humerus, of the humeral and ante- 
humeral (dark) stripes, in species which have the intervening pale 
stripe very narrow. 2nd. The presence or non-presence of the slender 
“ superior” basal yellow vitta of the tibiae. 3rd. The presence or non¬ 
presence of the narrow black edging of the occiput, noticed in 1862, 
as to the posterior lateral edging in vastus^ and observed for the first 
time in 1863, as to the superior edging of the occiput in that species. 
In the European Onychogomphus forcipatus^ however, the laborious 
researches of Dr. Hagen (Mon. Gomph. pp. 33—40 and Plate 2) 
have shewn that there are several remarkable geographical variations, 
with transitions from one to the other, and where we should least 
expect it, in the % abdominal appendages. The Illinois species seem 
to have these parts nearly as uniform as a set of castings from the same 
foundry and the same mould.—Owing to my formerly mistaking 9 G, 
consohrinus n. sp. (described below) for the other sex of S G.fraternus^ 
I have erroneously stated in my Paper (p. 393) that the 9 oi/rater 7 ms 
has a long, slender, yellowish thorn at each end of the vesicle of the 
vertex. In reality fraternus 9 has not even the rudiments of any 
thorn there, while consohrinus 9 has just such a thorn as I have de¬ 
scribed. I may notice here a remarkable fact, of the truth of which 
I am satisfied, having observed it for three successive years, and 
which was the primary cause of the mistake just referred to. In the 
group uidgatissLTniis, to which belong fratermus, consohrmus and gras- 
linellus, and also in vastus which belongs to the group dilatafus, the 
% % are 4 or 5 times as numerous as the 9 9 . On the contrary, in 
my two closely allied species, fluoialis and amnicola^ the 9 9 are 2 or 
3 times as numerous as the S S . There is no possibility of mistake 
in the latter case, as my specimens have been solely obtained from in¬ 
dividuals just crawling out of the pupa, and during a long period of 
time in each year. So far as regards the group vidgatissimus the fact 
is confirmed by what can scarcely be a mere coincidence. In that 
