ISOPODS OF NORTH AMERICA. 535 



five segments are modified, the last three joints being twisted to one 

 side. Extensor muscles enormously developed. 



"The abdomen consists of six fleshy segments, five of which bear a 

 pair of pleopoda. 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 ventrally. This ven- 

 tral ramus is broadly expanded in the pleopoda of the first abdominal 

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

 keels similar to those on the las^ thoracic segments. Between the 

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



Description of adult male. "Three and two-thirds longer than 

 broad. Abdomen unsegmented, about a third of entire length. Color 

 dull yellowish. Around ths heart in the abdomen 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 about 3 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 antennula?. From the depres- 

 sion the eight -jointed antennae and the conical rostrum take their 

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

 distal ones bristle-tipped. Sixth, seventh, and eighth joints very 

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

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

 phan r nx. Apex of latter conceals tips of mandibles and median part 

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

 tips. I have not found maxillula?. Between the maxilla? and extend- 

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

 The thorax consists of seven fleshy segments. It narrows slightly 

 posteriorly and is moderately convex. Sides subparallel, somewhat 

 deflexed, 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. Abdomen 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- 

 la? 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. 



l. U. S. Fish Commission, XXI, 1902, pp. 53-56. Consult this reference also 

 for description of immature forms. 



