256 GEOL, AND NAT. HIST. SURVEY OF MINNESOTA. 
Pleuroxus bairdii Schoedler. 
Pleurovus trigonellus—Baird. 
This form, so far as can be gathered from Baird’s brief description 
and figures, differs from the others in having the shell marked by 
straight parallel lines running diagonally back and upwards, and in 
lacking one of the terminal bristles on the 5-setose ramus of the an- 
tenne. The first is a possible but unusual structure, while the second 
might result from an overlooking of the very small seta which fills this 
place in the other forms. Baird himself did not distinguish it from 
P. trigonellus. 
*Pleuroxus denticulatus Birge. 
PLATES XLV, Fic. 8; LXIII, Fies. 10a, 12, 13. 
Resembling very closely P. adunctus, which, however, has a broader 
post-abdomen than the ordinary P. denticulatus. The posterior angle 
of the shell is armed with from one to four (generally three) teeth. 
The beak is very long. 
The character most emphasized by Birge is a series of teeth along 
the anterior margin of the valves. The same thing is found in P. pro- 
curvus, aS I have repeatedly satisfied myself. In certain positions 
these teeth do not show, or the smaller teeth on the lower margin only 
appear. P. adunctus, as figured by Schoedler, has similar teeth on the 
lower margin, and very likely has them anteriorly. The edges of the 
valves are heavily fringed with pectinate setze. The male has a shorter 
beak and the post-abdomen simply rounded without the peculiar 
modifications seen in P. adunctus. 
There seems to be two varieties in Minnesota, both of which have 
the characteristic irregular striations, which radiate from an irregu- 
larly marked or unmarked area in the centre of the shell toward the 
edges; both have the toothed posterior angle and the serrated posterior 
angle and the serrated anterior margin. But the common form is much 
longer, with the dorsal margin less convex and the beak shorter. The 
robust form has a larger pigment fleck, while the post-abdomen is 
shorter and more robust, resembling more nearly Schoedler’s figures 
of the abdomen of P. adunctus. There is another variation or abnor- 
mality, in which the lower margin is quite concave. The resemblance 
to P. procurvus is remarkable in some phases. 
I have collected this species in Blount springs, Ala., in the St. 
Croix river, and at various intermediate points, as well as very often 
in Minnesota. 
