302 The American Naturalist. [April, 
antennule about equals the distance between the eye and the 
pigment fleck. Antenne are short, small, with long sete. 
The specimens do not agree in all respects with the descrig- 
tion given by Birge of Pleuroxus gracilis var. unidens, but do 
agree in many points. The largest specimen found measures 
.60 mm. in length by .38 mm. in height; another measures 
.60 mm. long and .33 mm. high. Birge gives a measurement 
of .85 mm. by .46 mm. and states that the species is the largest 
yet seen. The original description of P. gracilis var. unidens 
states that, “the striation is very plainly marked. The spec- 
imens found by the author are only very faintly striated and 
that most distinctly at the anterior part of the shell where the 
lines of striation are approximately parallel to the anterior 
margin. The larger part of the surface is free from markings, 
either striation or reticulation as far as could be observed. 
The 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 Her- 
rick. 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 spec- 
imens, and the description and drawings of P. gracilis var. uni- 
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 
strie. The dorsal margin is convex, the hinder margin nearly 
straight. Its lower angle is rounded and is without teeth. 
The lower margin is concave and has long plumose sete. The 
front margin is strongly convex. The postabdomen is long 
