ABT. 5. NORTH AMERICAN PARASITIC COPEPODS WILSON. 35 



25. Third legs covering the genital segment, the abdomen, and nearly all of 

 the fourth legs; cephalothorax wider than long; free thorax with projec- 

 tions at anterior corners 3 mm., female, larvatus Heller, 1865 



25. Third legs covering only bases of genital segment and fourth legs, the rest 

 free; cephalothorax much longer than wide; free thorax without projec- 

 tions 3 mm., female, lativentris Heller, 18G5 



25. Third legs covering entire ventral surface; cephalothorax wider than long, 



lobed on the lateral margins; free thorax with processes at anterior and 

 posterior comers 5 mm., female, percis Thomson, 1889 



26. Cephalothorax narrowed anteriorly; dorsal plate with a shallow posterior 

 sinus 3.50 mm., female, frondeus Wilson, 1913 



26. Cephalothorax widened anteriorly ; dorsal plate with a deep median pos- 



terior sinus 2 mm., female, obscurus Wilson, 1913 



27. A plate on the ventral surface similar to the dorsal plate and covering the 



bases of the third and fourth legs 28 



27. No ventral plate; the third and fourth legs, the genital segment, and the 



abdomen entirely visible in ventral view 29 



28. Posterior margin of dorsal plate deeply bilobed ; the genital segment and 



abdomen entirely concealed 4.50 mm., female, temminckii Nordmann, 1864 



28. Posterior margin of dorsal plate pointed ; the genital segment and abdomen 



visible ventrally 10 mm., female, petersi, Beneden, 1857, p. 36 



29. Fourth legs projecting their entire length; their rami the same diameter 



throughout and squarely truncated at their tips ; third legs uniramose, 



5 mm., female, nordmanni, new name, p. 47 



29. Fourth legs projecting their entire length, their rami acuminate ; third legs 



biramose, rami acute 8.70 mm., female, paradoxus (Nordmann), 1832 



29. Fourth legs not reaching posterior margin of dorsal plate; cephalothorax 



with a deep incision on either side near the anterior margin, 



6 mm., female, pupa Burmeister, 1833 



30. Fourth legs undivided and broadly foliaceous 31 



30. Fourth legs undivided and cylindrical 32 



30. Fourth legs divided half their length or less 33 



30. Fourth legs divided to their base 36 



31. Fourth legs pointed at their tips and unarmed ; third legs narrow and 



cylindrical 1.85 mm., male, pagelli Kr0yer, 1863 



31. Fourth legs broadly rounded at their tips and armed with spiny processes, 



2 mm., male, lappaceus Wilson, 1913 



31. Fourth legs emargiuate at their tips, leaving two knobs armed with short 

 sinues 2 mm., male, chlamydotus, new species, p. 48 



32. Cephalothorax narrowed and rounded anteriorly ; fourth legs slender, 

 bluntly rounded, and nearly as long as the entire body ; third legs much 

 shorter 2 mm., male, oiscurus Wilson, 1913 



32. Cephalothorax narrowed and rounded anteriorly ; fourth legs stout and 

 acute at the tips and much shorter than the body ; third legs one-third 

 as long as the fourth 2 mm., male, pomatomi Rathbun, 1887 



32. Cephalothorax broad and squarely truncated anteriorly; fourth legs stout, 

 bluntly rounded, much shorter than the body ; third legs mere stumps, 



very short 1 mm., male, Icidyi, new species, p. 40 



33. Cephalothorax narrower and shorter, than the free thorax 34 



33 Cephalothorax wider and longer than the free thorax 35 



34. Rami of fourth legs broad and bluntly pointed ; endopod of third legs much 

 shorter than exopod 3 mm., male, k'r0yeri Beneden, 1851 



34. Rami of fourth legs narrow and acuminate; rami of third legs subequal, 



3 mm., male, gisleri Beneden, 1852 



