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reverse. The valves differ widely. In P. hastatus the head is 

 very small, the rostrum slender and strongly curved, and the 

 post-abdomen tapers gradually to a point. In P. straminius the 

 head is longer, the rostrum short, blunt, not much curved, and the 

 abdomen shorter and truncated at the end. 



Cambridge, Mass., 1875 ; common. Not found in Madison, 

 Wis., where its place seems to be taken by P. procurvus and P. 

 denticulatus, which are far more common there than in Cam- 

 bridge. 



The name is given on account of the color. 



SPECIES 3. 

 Plate II. Fig. 12, 



Pleuroxus Insculptus. sp. nov. 

 Length, circ. 0.27 mm. Height, 0.18 mm. 



Valves strongly marked by hexagonal reticulations. One tooth 

 at lower posterior angle of valves. 



This is by far the smallest species that I have seen. The dor- 

 sal margin is little arched, so that the hinder margin is not much 

 shorter than the height of the valves. At the junction of the 

 posterior and ventral margins, there is a strong tooth formed by a 

 semi-circular incision in the posterior margin. Sometimes there 

 is a second very small tooth above it. The ventral margin of 

 the female is very slightly concave, the concavity lying in the 

 rear half of the margin. The shell is marked as in the pre- 

 ceding species, but the lines are much more distinct. At first 

 sight only the diagonal striae are "manifest. Closer inspection dis- 

 closes the true nature of the sculpture. The rostrum is rather 

 short, the fornices quite broad. The post-abdomen is short, broad, 

 truncated, with a dorsal row of eight or ten teeth. The ter- 

 minal claws are serrated and have the usual . two basal spines. 

 The last (eighth) seta of the antennae is not always to be found. 

 The eye is quite large; the macula nigra much smaller. The 

 male is narrower proportionately. Its rostrum is short and 

 " stumpy." The post-abdomen is strongly concave below, dorsally; 

 with about the same number of teeth as in the female. The end 



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