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Gammarus fasciatus , Say. Apparently occurs throughout the state, 
in small rocky streams. Collected at Deer Park, La Salle county, in a 
small branch of the Vermilion, and in several streams in Jackson and Union 
counties. Scores of males and females were taken together under stones, on 
the 30th of July. 
In specimens from southern Illinois, the hands of the first pair in both 
sexes bear stout spines on the distal half of the posterior margin in addition 
to those on palm and at the tip of dactyl. A short transverse row of long 
hairs is situated at the base of the median palmar spine. The inner side of 
the hand of the second pair in the male is ornamented with two longitudinal 
series of short transverse rows of hairs, — the posterior of five rows, the an- 
terior of three. The palmar margin in the female has the lamellar edge. 
The lateral clusters of spines on the fourth posterior abdominal segment, in 
both sexes, each contain one very stout spine and several slender ones, 
while the median cluster consists of slender spines only. Each of the clus- 
ters on the fifth and sixth segments consists of two stout spines and several 
slender ones, except the median fascicle of the sixth segment, which consists 
of two distinct clusters of slender spines. The divisions of the telson have 
two clusters of hairs on the upper surface near the outer margin, of which 
the basal contains two spines. The spiny tips of the divisions are emarginate. 
Crangonyx gracilis , Smith. Very common in central Illinois. Col- 
lected at Bloomington, from slow, shallow streams. These specimens differed 
in several small details from those described by Prof. Smith, the most im- 
portant relating to the caudal stylets. In the typical form the tips of the 
three pairs are even ; but in the Bloomington specimens the second pair 
extends farther back than the third, and the first farther than the second. 
The inner ramus of the last pair is sometimes unarmed, but oftener bears 
one or two spines at or near the tip. The length of ovigerous females is 
10 mm. ; of the largest males observed 8 a mm. 
A form from southern Illinois represented in my collections by a few 
females, I cannot distinguish specifically from the above, although the second 
hands are proportionally longer and narrower and much more spiny, the 
anterior and posterior margins less convex, and the palmar margin more so. 
The tips of the caudal stylets reach the same perpendicular plane, and the 
inner ramus of the last is always as long as the width of the outer and bears 
one or two spines. 
Crangonyx mucronatus, Forbes. This remarkable species is perhaps 
entitled to rank as the type of a new genus ; but, until I have the material 
for a more general study of its relations than I am able to make at present, 
I prefer to place it with its nearest allies in the genus Crangonyx. 
Colorless, blind ; length 9 to 10 mm, width ^ mm. The head, is a 
little longer than the first thoracic segment, its anterior margin concave at 
the bases of the upper antennae, convex between them ; the posterior mar- 
gin straight in the middle and curving forward on the sides. The front 
angles of the first thoracic segment are uncovered and produced a little for- 
ward ; the hind angles of the first five segments are rounded and produced 
strongly backward. The first three abdominal segments have the lateral 
