FRESH-WATER CRUSTACEA OP THE UNITED STATES. 655 



Auteuuuliie slii^litly more thaa half as long as the rest of the animal ; 

 first and second segments of the peduncle siibequal, ultimate segment 

 two-thirds as long as the penultimate ; flagellum in full-grown specimens 

 about once and a half as long, as the peduncle, and coipposed of about 

 twenty segments ; secondary liagellum about as long as the basal segment 

 of the primary flagellum, slender, and composed of ouly two segments, the 

 terminal one very short. Autenna3 only about half as long as the anteu. 

 nulae ; ultimate and penultimate segments of the peduncle elongated, sub- 

 equal in length ; flagellum a little shorter than the peduncle, composed 

 of seven or eight segments. Legs of the first and second pairs sub- 

 equal. 



Propodus in the first pair of legs nearly quadrate in outline, a little 

 longer than broad ; palmary margin nearly straight, with a few small 

 submarginal spines, each furnished with a cilium a little way from the 

 tip ; a similar stout spine near the posterior angle, and just at the angle 

 itself two short, stout, obtuse, and serrated spines; dactylus stout, 

 slightly curved, and armed with a slight tooth on the inside a little way 

 from the tip, and with a slender, setiform hair near the middle of the 

 outer margin. Propodus in the second pair more elongated than in the 

 first; the palmary margin somewhat oblique, and without the short 

 spines just at the posterior angle, but otherwise armed much as in the 

 first pair. Fifth, sixth, and seventh pairs of legs subequal in length ; 

 posterior pair slightly longest ; their squamiform basal segments with 

 the posterior margin serrate, and both margins armed with small spines. 



Postero-lateral angles of the first, second, and third segments of the 

 abdomen produced and terminating in a small tooth. Posterior caudal 

 stylets reaching to the tips of the penultimate ; the outer rami nearly 

 twice as long as the peduncle, and armed with a few slender spines; the 

 inner rami rudimentary, very minute, shorter than the diameter of the 

 outer, and wholly unarmed. Telson scarcely as long as the bases of the 

 posterior caudal stylets, slightly broader than long, and the posterior 

 margin with a triangular emargination, either side of which the extrem- 

 ity is truncate, and armed with three spines. Length, G""" to 7™™. 



Male.— The largest males seen from Lake Superior are considerably 

 smaller than the females, being about 5"^-° in length, and more slender. 

 In the first pair of legs, the palmary margin of the propodus is slightly 

 oblique, and armed each side with a submarginal row of about eleven 

 stout and obtuse spines, which are nearly equidistant from one another 

 except at the posterior angle, where about five of them are crowded 

 together, most of the spines with a notch and cilium a little way from 

 the tip. In the second pair of legs the propodus- is proportionally 

 shorter than in the female, and increases considerably in breadth dis- 

 tally, while the palmary margin is much more oblique, slightly arcuate, 

 and armed each side with a row of about, fifteen spines like those on 

 the first pair, but not so much crowded together at the posterior angle. 

 In other respects, the males resemble the females. 



