26 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



First fourteen pairs of legs or else all fifteen pairs with acute tibial 

 process. (Plate 4, fig. 8). Prefemur of legs with small pointed proc- 

 ess at distal end on dorsal side. Tarsi of all legs biarticulate. The 

 first tarsal joint increasing in size from first to last pairs of legs, in 

 the caudal pairs being very long and slender. All legs ending in three 

 claws. 



Coxal pores present on the last five pairs of legs. 



Both males and females occurring commonly under the same condi- 

 tions. 



Gonopods of female of type usual in family, the claw being large and 

 entire and serrate along dorsal edge (see Plate 4, fig. 4). 



Gonopods of male with ultimate article ending in a large, straight, 

 weakly chitinized process or claw the edges of which are finely serrate 

 (see Plate 6, fig. 8). 



TYPE. Z. dolichopus Chamberlin. 



The members of this genus are of medium size. The body in all 

 is conspicuously narrowed cephalad from the eighth segment and the 

 legs are relatively long, particularly the posterior pairs. Of the four 

 species at present known, three occur in the mountains of the western 

 part of the continent, the fourth occurring in the mountains of Virginia 

 and Tennessee. 



Key to subgenera and species. 



a. First fourteen pairs of legs with tibial spurs, none being present 

 on the anal pair; posterior angles of ninth, eleventh, and thir- 

 teenth or of these and the seventh as well, produced. 



Zygethobius Chamb. 



b. Body prevailingly brown, reddish pigment being absent 

 or nearly so; more loosely joined and relatively longer articles 

 of antennae beginning with the fifteenth. 

 c. Posterior angles of ninth, eleventh, and thirteenth dorsal 

 plates produced; articles of antennae 39 or 40; body 

 eight times longer than width of tenth dorsal plate. 



Z. dolichopus Chamberlin. 



cc. Posterior angles of seventh, ninth, eleventh, and thir- 

 teenth dorsal plates produced; articles of antennae 41; 

 body but about seven times as long as width of tenth 

 plate. Z. columbiensis, sp. nov. 



bb. Body brown to chestnut, reddish pigment commonly promi- 



