98 Wisconsin Academy of Sciences^ Arts, and Letters. 



which can hardly be anything else than a rudimentary sixth pair 

 of legs, although they are situated some way back of the fifth 

 pair. It was wanting in one of the dozen specimens which I ex- 

 amined, or at least I could not find it. If it is a rudimentary 

 sixth leg, this is the first case in which this structure has been 

 found in the Lynceinas. It confirms the opinion which I had 

 formed on other grounds, that Pleuroxus is the genus which 

 stands as the most generalized type of this sub-family. Color, 

 yellowish, transparent. Male not seen. 



Lake Wingra, Madison, Wis., Sept., 1877. Rare, only about 

 fifteen specimens found. 



This is the largest species of Pleuroxus yet seen, and P. stra- 

 minius seems to be the next in size, 



SPECIES 6. 

 Plate II. Figs. 13, 14. 



Pleueoxus Hamatus. sp. nov. 

 Length, 0.4-0.45 mm. Height, 0.21-0.25 mm. 



General shape like that of P. unidens. Valves marked by 

 oblique stride, and by short, irregular, horizontal striae. 



In general shape this species approaches closely to P. unidens, 

 though the back is somewhat more arched. The posterior mar- 

 gin of the valves is concave, the lower angle rounded, and en- 

 tirely without teeth. The valves are marked by striae running as 

 in P. denticulatus, and by short, faintly marked striaB, which run 

 nearly horizontally. These cross the oblique strias, and are found 

 all over the shell of valves and head. The species is, in mark- 

 ings, the third of a series. P. unidens has only stri^, and those 

 continued quite across the valves. P. denticulatus and P. pro- 

 curvus have strife at the edges and irregular markings in center,- 

 while the present species has stria) around the edges of valves, 

 and also the short markings all over them. 



The post-abdomen closely resembles that of P. denticulatus. 



The feet of the first pair in the female are furnished with a tol- 

 erably stout hook, of which a sketch is given in PI. II, fig. 14. 



