256 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



Pleuroxus bairdii Schoedler. 



Fleuroxus irigoneUus — 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- 

 tennse. The first is a possible but unusual structure, while the second 

 might result from an overlooking of the very small seta which lillsthis 

 place in the other forms. Baird himself did not distinguish it from 

 P. trigoncllus. 



* Pleuroxus tleuticulatus Birge. 

 Plates XLV, Fig. 8; LXIII, Figs. 10a, 12, 13. 



Eesembling very closely P. adunctus, which, however, has a broader 

 post-abdomen than the ordinary P. denticidatus. 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. j)>'0- 

 curvHS, 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. adimctus, 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 seta. 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. 



