IOWA ACADEMY OP SCIENCES. 
161 
shell is more arched dorsally than P. gracilis is figured by 
Matile. Birge’s description of P. gracilis var. unidens says: 
“The upper posterior angle is prolonged into a projection, 
quite characteristic, seen, I believe, in no other species.” In 
the specimens found there is a slight projection at the angle, 
but not so pronounced as figured by Birge and by Herrick. 
The lower posterior corner is rounded and has a small tooth 
anterior to it as in P. gracilis var. unidens. 
It seems improbable that the differences between the speci- 
mens and the description and drawings of P. gracilis var. imi- 
dens should fall within the range of variation of a variety. 
The males were not seen. Collected in small numbers in June, 
1895, from a shallow slough and a small creek. 
Pleuroxus excisus . — Only one or two individuals were observed. 
These were taken from Rat creek, a sluggish stream flowing 
into Lake Manitoba. 
Alonopsis latissima var. media.— The specimens resemble the 
species described by Birge, but have some points of difference. 
Birge’s description is as follows: “Rostrum prolonged, and 
shell sharp, somewhat quadrangular in shape, marked by striae. 
The dorsal margin is convex, the hinder margin nearly straight. 
Its lower angle is rounded and without teeth. The lower 
margin is concave and has long plumose seise. The front 
margin is strongly convex. The post abdomen is long and 
slender, resembling that of Camptocercus, and is notched 
at the distal extremity; it has two rows of fine teeth and some 
fine scales above them. The terminal claws are long, slender, 
with a basal spine in the middle, and are serrated. The anten- 
nules are long and slender, but do not reach to the end of the 
rostrum. They have each a flagellum and sense hairs. The 
antennae are small and have eight (|^?) se" ae and two (l-gg) spines. 
The labrum 'resembles that of A. leucocephalus, but is slightly 
prolonged at the apex. The intestine, caecum, and color resem- 
ble those of Acroperus. There is a trace of a keel present on 
the back.” 
Herrick’s statement, in part, is as follows: “The specimens 
seen in Minnesota resemble this species [A. latissima var. 
media] very nearly, apparently, but there are some differences. 
The terminal claw has an increasing series of spines to the 
middle; there seems to be no lateraJ row of scales beside the 
anal teeth; the abdomen is rather broad at the base and nar- 
rows toward the end. The shell is not square behind. The 
11 
