NO. 1900. TfEW SPECIES OF PARAS ITIC COPEPODS— WILSON. 237 



with the transverse groove considerably posterior to the center. Pos- 

 terior lobes long, narrowed posteriorly, and curved sUghtly inward, 

 projecting over two-thirds of the length of the fused second and thn-d 

 thorax segments, and bordered with an exceptionally wide transpar- 

 ent margin. . . 

 Eyes separated, placed well forward, and arranged m a semicu-cle, 



the median one posterior. 



Second and third thorax segments fused, with only a faint dorsal 

 groove to indicate the union, and no rounding of the anterior corners 

 of the third segment. These fused segments are three-fifths the 

 width and half the length of the carapace, with a good-sized lobe 

 projecting from the lateral margin on either side near the center, 

 which represent the lobes commonly found at the posterior corners 

 of the second segment in Nogaus forms. Fourth segment free, three- 

 fifths of the width of the fused segments, with both the anterior and 

 posterior corners rounded in the usual manner. 



Genital segment the same length and width, which is seven-ninths 

 the width of the fourth segment, narrowed anteriorly and posteriorly 

 with a shght protuberance on each lateral margin near the posterior 



end. 



Abdomen triangular, two-thirds the width of the genital segment 

 and nearly twice as wide as long; anal laminae small and semicir- 

 cular, each armed with four plumose setae of about the same length. 

 Appendages similar to those of the female, except for the usual 

 sex modifications. 



MaxiUipeds with a second large claw in place of the pad borne by 

 the female, the two claws shutting past each other hke a pair of 

 scissors. Swimming legs with plumose setae as well as spines and 

 with a small spine on the basal joint outside of each exopod. 



Total length, 3 mm. Carapace (including posterior lobes), 1.75 

 mm. long, 1.62 mm. wide. Fused segments, 0.62 mm. long, 1 mm. 

 wide. Genital segment, 0.5 mm. long and wide. Color (preserved 

 material), a uniform cinnamon browm, becoming lighter and yellowish 

 in the thinner portions of the carapace. 

 {pinguis, stout, plump.) 



This is the first male to be described for the genus Achtheinus, and 

 as it establishes a new genus type among the Nogaus forms we may 

 make for it the following diagnosis, corresponding to those already 

 established for other genera: * 



Genus Achtheinus (male).— Carapace large horseshoe-shaped, with- 

 out conspicilla or accessory lobes. Genital segment not enlarged ; two 

 pairs of legs visible dorsally; abdomen one-jointed, triangular; anal 

 laminge very small and semicircular; legs all biramose, rami of first 

 three pairs two-jointed, of fourth pair one-jointed; maxillte broad and 



> Proc. U. S. Nat. Mus., vol. 33, 1908, p. 446. 



