ISOPODS OF NORTH AMERICA. 535 



live seoments ar<^ inoditicd, tli(> last three joints being- twisted to one 

 side. Extensor muscles enormously developed. 



"Tlie abdomen consists of six lieshy segments, live of which bear a 

 pair of jdeopoda. Each pleopod has three oval blades arising- from 

 a short common base. Two of these are subequal and extend in a 

 lateral direction; the third is smaller and points ventr:dly. This ven- 

 tral ranms is broadly expanded in the pleopoda of the tirst abdominal 

 segment, especially on the right side. The lirst segment has ventral 

 keels similai- to those on the last thoracic segments. Between the 

 oval uropoda of the sixth segment is a minute conical prominence." 



Descriptimi of adiilf. male. — "Three and two-thirds longer than 

 ])road. A])domen imsegmen(ed,about a third of <'ntire length. Color 

 dull yellowish. Around th3 heart in the alxlomen is an orange-colored 

 area, and a narrow streak of same color runs forward along the mid- 

 dorsal line. Sometimes splashes of black occur on the sides of the 

 head and thorax. Length al)out 8 mm. 



"Head oval, elevated in center, the margin entire and not inflexed. 

 Eyes minute. On the under side is a shallow central depression, in 

 front of which arise the three-jointed antennuhe. From the depres- 

 sion tiie eight-jointed antenna' and the conical rostrum take their 

 origin. First joint of antenna' elbowed, the others cylindrical, the 

 distal ones ])ristle-tipped. Sixth, seventh, and eighth joints ver}'' 

 small, together scarcely equalling the fifth in length. Rostrum prom- 

 inent, built up dorsally by the labrum and ventrally l)v the hypo- 

 phar3"nx. Apex of latter conc^eals tips of mandibles and median^part 

 of labrum. Mandibles slender, with thick bases and sharp chitinous 

 tips. I have not found maxilluhe. Between the maxilhe and extend- 

 ing forward from a transverse ridge are the three-jointed maxillipeds. 

 The thorax consists of seven lieshy segments. It narrows slightly 

 posteriorly and is moderately convex. Sides subparallel, somew^hat 

 detiexed, epimera not distinct. First segment notched for reception 

 of head. Seven pairs of pereiopoda, whose structure and muscula- 

 ture can be understood by reference to the plate. Al)domen ovoid or 

 sometimes pear-shaped, shows no sign of segmentation, and has no 

 traces of appendages. "-^M. T. Thompson." 



89. Genus STEGIAS Richardson. 



Body of female with sixth segment of thorax not greatly longer 

 than any of the others. 



All six segments of abdomen distinct; lateral parts or pleural lamel- 

 he not developed. First three pairs of pleopods triramous; last two 

 pairs biramous. Uropoda consist of a pair of elongated lamella?, with- 

 out a conical process between the two. 



"Bull. U. S. Fish Commipi^ion, XXI, 1902, pp. 53-56. Consult this reference also 

 for descrijition of immature forms. 



